How to get a unique value based on multiple columns a user picks?

I have followed the explanation here on special values. I have also looked at the sample app. However I do not see a way by which when I create a row - for e.g. in that app - creating an expense and I pick a Trip, How do i automatically get the unique value in the expense row ? I cannot ask the user to pick the unique value - the user would only pick an entry they know - like say a Trip Name.

In this case - I have 2 columns - Name and Trip and for the Name Trip Combination there is a uniqueid. I would like to have every expense row connected to a the Name Trip Combination unique ID. the User can pick the name and the trip but how do i derive the unique id from this composite key that is based of 2 columns

If you have a row somewhere that contains a unique ID, a name, and a trip, I would let your user navigate to details screen if that name/trip in the app. I would then add a form button on that screen for expenses. The form can automatically pull in the unique ID from the name/trip details and write it to the sheet. All of the available columns are shown along with the regular components on the right side of the builder screen.

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