Auto-Complete is not support in Go. http://help.filemaker.com/app/answers/detail/a_id/7748/kw/auto-complete%20FILEMAKER%20GO
The only choice you have is to create your own and use another field or a portal to display your results. You would use a script trigger to capture keystrokes and update another field or portal. Go doesn't support plug-in either.
You can always request this feature at the following link.
I was hoping there would be a work around with script triggers somehow.. :(
There is a work around but you would have to use a field or portal to display your list. All a script trigger is going to do is capture keystrokes and create a list to display in a field or portal. It Is not going to make the value list show a list, that is not supported. I guess if it was that easy Filemaker would have included support for Auto-Complete in Go.