Cannot add record to multi-select lookup

Cannot add record to multi-select lookup

Hi,

I have creates multi-select lookup "References_Account" in contacts record linked to accounts.

I an trying to add multi-select lookup to a record. The flow executes successfully but field is not updated. I tried below function code as well, didn't worked.

void updateLookup(int accountid, int contactId)
{
mp = Map();
mp.put("References_Account",accountid);
updateResp = zoho.crm.updateRecord("Contacts",contactId,mp);
}