You can get the user ID from the WP Users page. You can get the Event Espresso Contact ID from the Event Espresso > Registrations > Contact list admin page.
In this example, the user ID is 18 and the Event Espresso Attendee ID is 862.
Available here: Debug Bar Debug Bar Console
You click on the Debug Bar link, then Console add add the following code, replacing the User ID and Meta Value.
<?php
global $wpdb;
$user_id = 18;
$meta_key = $wpdb->prefix . 'EE_Attendee_ID';
$meta_value = 862;
add_user_meta( $user_id, $meta_key, $meta_value, true );
?>
Then you click Run.
Hey Josh! I am trying this and not having any luck. The Debug Console has not been updated in over 2 years, so that may be the issues. When I click "run", it does nothing. I looked at inspect in Google and under console had a yellow warning: jquery-migrate.js:45 JQMIGRATE: jQuery.fn.unload() is deprecated. Any other suggestions? Also, I have about 150 to do. I can easily merge the $user_id and $meta_value to get a huge string of commands. Is that doable or do you have a suggestion for a better solution. Many thanks!