Property definition automatic values - Value list - Multiselect

You need to use Lookups rather than just IDs in your SetValue method. You might also consider using SetValueToMultiSelectLookup.
See https://developer.m-files.com/APIs/COM-API/Reference/#MFilesAPI~TypedValue~SetValueToMultiSelectLookup.html

Something like this should work (not tested)

Dim oObject, oObjects
Set oObjects = <some code that identifies the objects you want to pull the list from>
Dim SelectedValue : set SelectedValue = CreateObject("MFilesAPI.Lookup")
Dim SelectedValues: Set SelectedValues = CreateObject("MFilesAPI.Lookups")
For Each oObject In oObjects
    SelectedValue.Item = oObject.Item
    SelectedValues.Add -1, SelectedValue
Next
Output = SetValueToMultiSelectLookup(SelectedValues)

Thanks so much. That worked a treat!