I have a dropdown on an Create record action interface that utilizes a related table for its values. On the Update record action interface I need to have the same dropdown in case it needs to be updated, however I need it to retain the selection that was made when the record was added. I am struggling to setup that connection on the Update interface.
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Generally speaking, you can pre-populate the value of a dropdown simply by setting its "value:" parameter to the value of the corresponding entry in its choiceValues list. Presumably you will have saved the original selection to the DB when your record was created, so upon update, you will be feeding in the previously-created data and establishing the initial state of your interface based on that.
Any more detail than this is hard to give or even guess at without some detailed feedback with respect to what you've tried so far, with respect to your current process setup and what you're trying on-form, in addition to what exactly isn't working for you thus far.