Option to have Sales Person Be Currently Logged User than always Account Manager Merged

Currently the sales person for an Opportunity / Quotation is defaulted to the Account Manager.

For some customers an account manager will look after customers in an area and be assigned as the account manager, but any of the sales rep can raise sales for those accounts and get the credit for the sale. So rather than always attributing the sale to the account manager, it would be in this scenario the sales rep it should attributed to (i.e. the user raising the Opportunity / Quote).