WordPress REST API Custom Endpoint with URL Parameter

First you need to pass in the namespace to register_rest_route

Like this

add_action( 'rest_api_init', function () {    register_rest_route( 'namespace/v1', '/product/(?P<id>\d+)', array(        'methods' => 'GET',        'callback' => 'my_awesome_func',    ) );} );

Your name space namespace/v1 and your route is /product/{id} like this /namespace/v1/product/81838240219

and now you can use the route inside your function like this

function my_awesome_func( $data ) {    $product_ID = $data['id'];}

If you need to add options for ex./namespace/v1/product/81838240219?name=Rob

and use it inside the function like this

function my_awesome_func( $data ) {    $product_ID = $data['id'];    $name = $data->get_param( 'name' );}

The process is very simple but requires you to read this documentation

I modified the provided answer a little to get my desired endpoint:

/wp-json/api/v1/product?identifier=81838240219

add_action( 'rest_api_init', function () {register_rest_route( 'api/v1', '/product/', array(      'methods' => 'GET',      'callback' => 'ea_get_product_data',    ) );} );function ea_get_product_data( $data ) {    $identifier = $data->get_param( 'identifier' );    return $identifier;}