35% is illegal to use even by a dentist in this country it is many times over the safe, allowed limit in U.K. , EU , NZ etc.
It is very,very,very likely to cause most people extreme sensitivity and for some, permanent damage .
Really not a safe suggestion and very illegal to sell to public or dentists in this country.
Night retainers are not suitable for bleaching as they do not have reservoirs like whitening trays will have. In other words the solution will squirt onto gums rather than being held against teeth . See a dentist for safe, properly fitting whitening trays.