Thank you for your post.
In FileMaker Pro, you can tab to a Popover button, but you must press return to have the Popover appear. FileMaker Go will not access a Popover button. Therefore, you should attach an OnObjectExit script trigger that selects the first field in the portal Popover. This will work with both FileMaker Pro and FileMaker Go (no extra return is needed to access the Popover).
However, once you are in the Popover, pressing tab will only remain within the Popover. That is, if you have two fields in the Popover, tabbing will only take you back and forth between the two fields. Again, if you want to exit the Popover, attach another OnObjectExit script trigger that selects the next field outside the portal. This way, you can tap through the portal, including the Popover, and then return to the next desired field.
Let me know if you need additional clarification.