Convert tags to custom text field w smart rule

I’ve imported a lot of data from Paperless to Devonthink - Paperless fields are imported as tags.

No I want to create a smart rule converting the tag “amount=51,12 €” (51,12 being the variable) to a custom text field showing “51,12 €” and then delete the tag.

But: In the smart rule dialog I cannot scan tags, I can only scan name and text.

Is there a way to convert a tag to text, so it can be scanned for strings - without havin to create a script?

Thanks