So i want to save the result of DLookup in another field on same form and that is actually a record source of a main table.
First, you can't have one form that has the record source of both a query and another table. You must find another way to do that. You could use a subform, but it sounds like your are trying to do something very easy in a very difficult way.
How are the table and query related? Do they have a matching index that you can use to find the proper field in the table?
What is the purpose of these actions, so we can understand what you are trying to do and offer different options/solutions that would fit your needs.