How to stop offering to autofill a specific field on a website

The field I wrote about (see attached below) is exposed after logging into my country club site, which is www.pecangrovesmga.org/signup.php. 

Since this field calls for a user name, 1PASSWORD wants to use the login's username after it already logged in using it.  I have previously learned (one of your suggestions) I can click the down arrow and type what I want, but the down arrow is so small (on my iPhone) that I have often aimed wrongly and thus registered an incorrect response.  With no way to correct it (of course a shortfall of the very simple app), I have to contact someone personally to make the correction.

In a perfect world, the ideal would be to expose a user-defined selection list and select the one I want.  That may not be possible in 1P but can you advise something close to the ideal?