Setting a value on existing tag metadata field when that tag was just created by a user

to add a meta value on a term creation, you can use that :

const TAXONOMIE_SLUG = "category";

add_action("create_" . TAXONOMIE_SLUG, function ($term_id, $tt_id) {

    $user = wp_get_current_user();

    update_term_meta($term_id, "creationUser", $user->ID);

}, 10, 2);