How to Populate Dropdown with Selected Values from Multiselect field

How to Populate Dropdown with Selected Values from Multiselect field

I am trying to use the selected values in a multiselect field in one form to populate the dropdown values in another form when the records match.  3 forms in all are linked.

For example, if I have a multiselect field called "MM" in a form called "FormA" and a dropdown field (called "DD") in a form called "FormC".  FormB contains a link field called SubID.

FormA has fields:
ID
MS

FormB has fields:
ID
SubID

FormC has fields:
SubID
DD

So if I select a SubID in FormC I find the record in FormB where the FormC.SubID=FormB.SubID.  Then I find the record in FormA where the FormA.ID=FormB.ID.

But I can't get the selected values in the MS field for the particular ID record to populate the DD dropdown in FormC for the SubID record.

I the On User Input for the DD field with:

brec = FormB [SubID = input.SubID];
arec = FormA [ID = brec.ID];

I feel like I've tried every combination of for each and list functions thereafter, but nothing works.  My most common error is that the arec.MS doesn't exist.

Any help would be greatly appreciated.  Thank you.