You can do it via browser's tool. You need to copy cookies with all headers via Chrome browser > View > Javascript Console > Network > (right click)> Copy option menu > click select "Copy as cURL" :

Normally we curl in this way :

curl -c cookie.txt -d "LoginName=username" -d "password=changepassword" https://examplesite/acurl -b cookie.txt https://examplesite/b

Copy via right click will be very big (of course I changed things to prevent myself getting hacked) :

curl 'https://meta.stackoverflow.com/' -H 'pragma: no-cache' -H 'accept-encoding: gzip, deflate, sdch, br' -H 'accept-language: en-US,en;q=0.8' -H 'upgrade-insecure-requests: 1' -H 'user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36' -H 'accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8' -H 'cache-control: no-cache' -H 'authority: meta.stackoverflow.com' -H 'cookie: prov=xxxxxxxxxxx; __qca=P0-xxxxxxx-xxxxxx; acct=t=xxxxxxxxxxxx; _ga=GA1.2.xxxxxxxx; _gid=GA1.2.xxxxxxx; _ga=GA1.3.xxxxxxx; _gid=xxxxxxxxx9' -H 'referer: https://meta.stackoverflow.com/' --compressed

unfortunately, the login protocol is much more complex than that, and is not a scheme built-in to curl. this is not a job for curl, but some scripting language (like PHP or Python), though libcurl would be of great help to manage the http protocol and cookies and the likes. and libxml2 would be of help to parse out the login CSRF key, which is hidden in the HTML. and they may require a referer header, and they may even be checking that the referer header is real, not faked (idk, but it wouldn't surprise me).

first, make a plain normal HTTP GET request to https://sustainability.stackexchange.com/users/login , and make sure to save the cookies and the html response. now extract the POST URL and input elements of the form with id login-form, this includes the CSRF token, username, and password, and bunch of others. then make an application/x-www-form-urlencoded-encoded POST request to https://sustainability.stackexchange.com/users/login , with the cookies received from the first GET request, and the POST data of all the <input elements you extracted, and remember to fill out the "email" and "password" inputs.

NOW you should get the logged-in html, and to continue to get the logged-in version of the page, make sure to apply the same cookie session id to the next http requests (its this cookie session id that makes the website remember you as the guy that logged in on that account~)

here's an example in PHP, using libcurl and libxml2 (using PHP's DOMDocument as a convenience wrapper around libxml2, and using hhb_curl from https://github.com/divinity76/hhb_.inc.php/blob/master/hhb_.inc.php as a convenience wrapper around libcurl, taking care of cookies, referers, libcurl error handling (turns silent libcurl errors into exceptions, and more), at the end, it dumps the logged-in HTML, proving that it's logged in. (and the email/password provided, is a dummy account for testing, there's no problem in it being compromised, which obviously happens when i post the credentials here.):

<?phpdeclare(strict_types = 1);require_once ('hhb_.inc.php');$hc = new hhb_curl ( 'https://sustainability.stackexchange.com/users/login', true );// getting a cookie session, CSRF token, and a referer:$hc->exec ();// hhb_var_dump ( $hc->getStdErr (), $hc->getStdOut () );$domd = @DOMDocument::loadHTML ( $hc->getResponseBody () );$inputs = array ();$form = $domd->getElementById ( "login-form" );$url = $form->getAttribute ( "action" );if (! parse_url ( $url, PHP_URL_HOST )) {    $url = 'https://' . rtrim ( parse_url ( $hc->getinfo ( CURLINFO_EFFECTIVE_URL ), PHP_URL_HOST ), '/' ) . '/' . ltrim ( $url, '/' );}// hhb_var_dump ( $url, $hc->getStdErr (), $hc->getStdOut () ) & die ();foreach ( $form->getElementsByTagName ( "input" ) as $input ) {    if (false !== stripos ( $input->getAttribute ( "type" ), 'button' ) || false !== stripos ( $input->getAttribute ( "type" ), 'submit' )) {        // not sure why, but buttones, even ones with names and values, are ignored by the browser when logging in,        // guess its safest to follow suite.        continue;    }    // var_dump ( $input->getAttribute ( "type" ) );    $inputs [$input->getAttribute ( "name" )] = $input->getAttribute ( "value" );}assert ( ! empty ( $inputs ['fkey'] ), 'failed to extract the csrf token!' );$inputs ['email'] = 'vs5jkqyx4hw3seqr@my10minutemail.com';$inputs ['password'] = 'TestingAccount123';$hc->setopt_array ( array (        CURLOPT_POST => true,        CURLOPT_POSTFIELDS => http_build_query ( $inputs ),        CURLOPT_URL => $url ) );$hc->exec ();hhb_var_dump ( $inputs, $hc->getStdErr (), $hc->getStdOut () );

interesting note, by default, libcurl uses multipart/form-data-encoding on POST requests, but this site (and most sites, really), uses application/x-www-form-urlencoded-encoding on POST requests. here i used PHP's http_build_query() to encode the POST data in in application/x-www-form-urlencoded-format

The url for login is not https://sustainability.stackexchange.com/ it is https://sustainability.stackexchange.com/users/login

and the link you refer say

curl -u username:password $URL

not

curl $URL -u username:password

Try

USERNAME="mine@gmail.com"PASSWORD="myPassword"URL="https://sustainability.stackexchange.com/users/login"curl -u $USERNAME:$PASSWORD $URL

Update

Stack Exchange requires an additional key for login which is called fkey. If you inspected the login form from browser, you can see a hidden input field with name fkey and value is a hash value. It is required to identify the session and to prevent fake login attempts.