Subform : Recall existing data into subform - not create new data

My use case is that I have a multi-select field where I want users to be able to choose locations that already exist and bring them into a subform to edit. The subform is linked - not an inline one. The multi-select field and the subform are the "locations" form.

I need to be able to fetch and update existing records, not create new ones

I can get the first part of the code to return the record for the last option chosen in the multi-select. I just need it to populate (and thereby link) the existing location record.

//Use a null check to stop it running if there is nothing to match
if(location[ID = input.locations].ID != null)
{
varLocations = input.locations;
//create and clear the list as it's run every time something is added to the multiselect
varCollection = Collection();
varCollection.clear();
//Add all the selected locations to the collection
varCollection.insertall(input.locations);
//Get the size of the collection (minus a one because it's a zero index)
varLocationCount = varCollection.size() - 1;
//Now we just need the ID of the last location chosen
varLastLocation = varCollection.get(varLocationCount);
//
//Fetch the location about the chosen location
varFetchLocation = location[ID == varLastLocation];
//
// Here's where I try to add the fetched location to the subform
for each row in input.SUB_LOCATIONS
{
row.ID = varFetchLocation.ID;
row.location_name = varFetchLocation.location_name;
}
}

Here's a visual example of my desired end result

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