Do you have more than one portal on this layout?
If so, then you need to put the focus on the correct portal before your script executes the Go to Portal Row step.
Use the Name Box in the Inspector's position tab to give your portal an object name.
Then Put a go to object script step that goes to that object name just before the go to portal row step.
That should at least put the focus on the correct portal row.
IF that doesn't resolve the issue or you only have one portal, look for any script triggers that might be tripped when your script returns to this layout or when your script puts the focus in the portal row of the pop up field. Such a script might change the focus (such as using Commit Records) in such a way as to keep this from working.
But I don't think that you can get a pop up menu to deploy auto-matically from a script.
A drop down list formatted field--without the drop down arrow, on the other hand, will deploy the value list automatically when the script puts the focus on that field.
Perfect. I don't know what I'd do without your help. This thing is finally coming together.