The Simple and Correct Way to Add User Meta

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Trying to detect when a UI was triggered, is indeed the wrong general approach (unless you want to tie to the UI). A proper one will be to hook on the user_register action.

add_action( 'user_register', 'wpse223196_registration_save', 10, 1 );

function wpse223196_registration_save( $user_id ) {

   add_user_meta($user_id, 'score', 5);

}