I do it normally this way:

Call M&M SEN Hotline and provide the SWISS Booking Code and ask for the Amadeus Booking Code. Important: If the Agent doesn't know directly what to do and doesn't provide you the code directly, re-dial and hope for an Agent knowing what to do (one time the Agent was not sure what to do and f**ked up the Amadeus part and so i'd to call SWISS hotline and never saw the correct booking at M&M Portal or with "CheckMyTrip" ).

With the Amadeus Code you can add your booking to the M&M Page under "My Bookings". If you are still not able to reserve seats in eco as SEN, call Hotline again (then your M&M Number is not correctly added and it'll not work just to add it via M&M Portal - the Agent has to correct it).