Send UserId to mySql

I have a text area that when I hit a submit button it send data to a mysql table through php with a timestamp. I am trying to have the userId of the individual who submits insert as well so that I can keep track of who is send me information. I’ve attempted to use the <?php echo S2MEMBER_CURRENT_USER_ID; ?> but all I receive is S2MEMBER_CURRENT_USER_ID. I am fairly stuck on what to do next.

https://wordpress.org/plugins/s2member/