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Let make it a smallest blog possible.
below code i have been using without any issue when it comes to setting a partylist or multi lookup in CRM. In the below code i am retrieving the contact id or the id of entity which is there in the regarding field & setting it on the “From” field.
With slight modifications you can use the code on any other activity if you need to set partylist.
Call this function on Onload of the record or on onchange of the regarding field.
hope this helps!