I've figured it out. The easiest way is to add a domain that filters what you need. Because it doesn't allow to equal to a field, you have to edit after this the XML file and remove the quotes. Be aware that entering the domain again will put the quote back in, so you will need to remove them again. I can't get why Studio by default only allows Numbers or Strings to eval, when only removing the quotes makes this tool so much useful.

Thanks anyways.