Just Contact LNER (The operating traincompany) via Mail or on Facebook and say which train on which day and you have a Railpass usually they answer within minutes :) 

The reservations should be free of charge for this train :) 

Or-as there are trains about ev 30 mins (some fast, others a bit slower), just do it when in LON at station or when you arrive LON. I guess you are flexible enough to take one earlier or later  in the highly unlikely case this train may be fully booked.

This ryle has never been and is in fact a remnant of covid-distancing rules to prevent overcrowding, it may well be scrapped if it is for only next month or so.