I thought this would be easy to look up how to do, but can't really find any good documentation or even see the control when I create a form. Is this available with Nintex Online? Is there an example on how to do this?
umm good point, I'm not sure if it's available in SharePoint Online version, I guess if you can't find it it isn't.
Only other options I can think of right now are custom code (query the REST api and display on the form) or embed the form on a page in a web part, and have another list web part below with a search box