I am guessing with “sign up pixel” you mean the event code. This is how I do it:

1. 
   

   Add the universal website tag to the javascript section of your main page (in the tag)

   
   


2. 
   

   Then go to the form confirmation dialog and add the universal website tag there as well (also in the tag).

   
   


3. 
   

   To track sign up events as conversions, paste the twitter event code after the universal website tag in the form confirmation dialog.

   
   

This will ensure that website visitors are tracked (1), the tracking library is present when the form confirmation dialog is displayed (2), which allows the event code to be fired (3).