Hi @andrewnewby
I’m assuming you that by “sing up form” mean “subscribe page”. That’s how you will find it on your phpList installation. On this page on the manual you can find a lot of useful information regarding subscribe pages: https://www.phplist.org/manual/books/phplist-manual/page/creating-a-subscribe-page
Also, you can check this older thread from the forum where subscribe pages are being discussed: [SOLVED] Ajax subscribe api
Hope the above help!
Mariana