Warning: Cannot modify header information

Well, the error says exactly what’s going on.

You’re outputting content, before the data is finished being manipulated. It’s like you’re trying to echo something but WordPress still has to do some stuff and it’s telling you “No, no, let me do my thing first”. You’re calling the echo too early.

In short, you’re 99% outputting before wp_head.

So, go to header.php and after the body div, put your

WPCFSSearchForm::show_form($preset,"preset-$id");

Or

require_once preset-display.php

Should work.

Thing is, this is really bad to do, so I suggest hooking show_form to an action and outputting it anywhere you want.