You misunderstand, it's not that an additional variable is needed. It's that the variable that exists when the page loads needs to also exist when the form submits.
I see two recordsets on your page:
rsSubjects - uses the ClubID URL variable as a filter
rsEnq - uses the SubjID form element to filter.
I also see that there are 2 forms on this page, looks like the first form filters the rsEnq recordset, then you submit the second form.
This method will not work for you because of the way forms work.
The information form the first form will not be available when the second form submits.
Form data is only available to the action page while it is loading, once the actin page finishes loading, the form collection is deleted by the server.
You will need to do merge the 2 forms into one so that the SubjID form element is part of the form that sends the email.
you will also need to edit the action of the form:
action="<?php echo (htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)); ?>"
action="<?php echo(htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)."?".$_SERVER["QUERY_STRING"]); ?>"
this is need so that the ClubID URL variable is passed when the form submits.