Answer by janh for curl POST work with user meta but not the custom user meta

You'll get the value as the first parameter, or you could get it out of $request, which is a WP_REST_Request object. Since you're using POST, the parameters out of $request->get_body_params() which will return an array. If you just need the actual meta value, you don't need to do this.

Your callback function should look something like this:

function user_meta_update_callback( $value, $user, $field_name, $request ) {    update_user_meta( $user[ 'id' ], $field_name, $value );}