Lets say you want to add recordLink for Customer 10000
Then
Cust.GET('10000');
RecRef.GETTABLE(Cust);
RecordLink.INIT;
RecordLink."Record ID" := RecRef.RECORDID;
.....
RecordLink.INSERT;
Where Cust is record type variable Customer
RecRef is RecordRef
RecordLink is record type variable Record Link