Try 1. Edit .htaccess

sudo a2enmod headers
sudo service apache2 reload

.htaccess

<IfModule mod_headers.c>
  Header set Access-Control-Allow-Origin "*"
  Header set Access-Control-Allow-Methods: "POST, GET, OPTIONS, PUT, DELETE"
  Header set Access-Control-Allow-Credentials: true
  Header set Access-Control-Allow-Headers: X-Requested-With
</IfModule>

You can use add rather than set, but be aware that add can add the header multiple times, so it’s generally safer to use set.

https://www.w3.org/wiki/CORS_Enabled

Try 2. Edit functions.php

Put this in functions.php

// Enable CORS for the API
// https://gist.github.com/miya0001/d6508b9ba52df5aedc78fca186ff6088
// https://github.com/ahmadawais/WP-REST-Allow-All-CORS/blob/master/plugin.php
function my_customize_rest_cors() {
  remove_filter( 'rest_pre_serve_request', 'rest_send_cors_headers' );
  add_filter( 'rest_pre_serve_request', function( $value ) {
    header( 'Access-Control-Allow-Origin: *' );
    header( 'Access-Control-Allow-Methods: GET, POST, OPTIONS, PUT, DELETE' );
    header( 'Access-Control-Allow-Credentials: true' );
    header( 'Access-Control-Expose-Headers: Link', false );
    header( 'Access-Control-Allow-Headers: X-Requested-With' );
    return $value;
  } );
}
add_action( 'rest_api_init', 'my_customize_rest_cors', 15 );

Try 3. Edit Appache virtual host .conf file

Add the header inside the <Directory> section in the Apache virtual host files for the domain. (You may have 2 of them, one for SSL and one non-SSL)

# Allow .htaccess and Rewrites
<Directory /var/www/mysite.com/public_html>
  Options FollowSymLinks
  AllowOverride All

  # Enable CORS
  Header set Access-Control-Allow-Origin "https://www.mysite.com"
</Directory>

The side benefit of editing the .conf file and not .htaccess is that i can be sure that i don’t have to worry about the order of my redirects and what section goes before what

In the end none of these worked for my user case because i was also setting up redirection for the domain. When you redirect a domain, the origin gets changed to null which is not an acceptable Origin