How to add a custom attribute?

Find the name of your field.

[text* text-21]

If the name of your field name="text-21", like in my example, add this code to functions.php file.

add_filter( 'wpcf7_form_elements', 'imp_wpcf7_form_elements' );function imp_wpcf7_form_elements( $content ) {    $str_pos = strpos( $content, 'name="text-21"' );    if ( $str_pos !== false ) {        $content = substr_replace( $content, ' data-attr="custom" data-msg="Foo Bar 1" ', $str_pos, 0 );    }    return $content;}

Note, it will add those attributes to all forms elements where the name is text-21, if you want prevent it then give your form element some unique name [text* unique-name]

And change the code to

add_filter( 'wpcf7_form_elements', 'imp_wpcf7_form_elements' );function imp_wpcf7_form_elements( $content ) {    $str_pos = strpos( $content, 'name="unique-name"' );    if ( $str_pos !== false ) {        $content = substr_replace( $content, ' data-attr="custom" data-msg="Foo Bar 1" ', $str_pos, 0 );    }    return $content;}

Multiple attributes can be added also. eg

add_filter( 'wpcf7_form_elements', 'imp_wpcf7_form_elements' );function imp_wpcf7_form_elements( $content ) {    $str_pos = strpos( $content, 'name="your-email-homepage"' );    $content = substr_replace( $content, ' aria-describedby="emailHelp" ', $str_pos, 0 );    $str_pos2 = strpos( $content, 'name="your-fname-homepage"' );    $content = substr_replace( $content, ' aria-describedby="fnameHelp" ', $str_pos2, 0 );    $str_pos3 = strpos( $content, 'name="your-lname-homepage"' );    $content = substr_replace( $content, ' aria-describedby="lnameHelp" ', $str_pos3, 0 );    return $content;        }

Here is a generic solution that doesn't involve hardcoding the field name and the attributes

add_filter( 'wpcf7_form_tag', function ( $tag ) {    $datas = [];    foreach ( (array)$tag['options'] as $option ) {        if ( strpos( $option, 'data-' ) === 0 ) {            $option = explode( ':', $option, 2 );            $datas[$option[0]] = apply_filters('wpcf7_option_value', $option[1], $option[0]);        }    }    if ( ! empty( $datas ) ) {        $name = $tag['name'];        $tag['name'] = $id = uniqid('wpcf');        add_filter( 'wpcf7_form_elements', function ($content) use ($name, $id, $datas) {            return str_replace($id, $name, str_replace("name=\"$id\"", "name=\"$name\" ". wpcf7_format_atts($datas), $content));        });    }    return $tag;} );

It works on all data attributes so you can use it like this

[text* my-name data-foo:bar data-biz:baz placeholder "Blabla"]Output: <input type="text" name="my-name" data-foo="bar" data-biz="baz" placeholder="Blabla">

Since wpcf7 doesn't provide a way to hook into options directly the solutions uses a trick and temporary replaces the name of the field by a unique id that is then replaced in a later filter with the correct name and added attributes

If you need it to work with more than just data attributes you can whitelist some more attributes by replacing this line

if ( strpos( $option, 'data-' ) === 0 ) {

to something like the following

if ( preg_match( '/^(data-|pattern|my-custom-attribute)/', $option ) ) {