I've verified this yesterday already, this is all you need (and ofcourse another UI Policy for the devices):

Thank you so much Mark. To clarify my confusion regarding misconfiguration.

Also see this article I wrote 5 years ago:
https://www.servicenow.com/community/developer-articles/clear-the-field-value-clear-the-variable-val...

Hi @Yogita11_ ,

In Ui Policy you have options as applies to catalog item, applies to Requested Item ,in that check on catalog item alone so that it will work only on portal

Please mark my answer helpful/correct if it resolved your query

Thanks