How to get checkbox in code template to have multi select

how are you displaying your questions?

you can create a valuelist put them right in the list in blaze or create a generic list provider and pull them from a class outside of blaze like from a db or whatever.

once you have the list in a provider then in your template add a valueholder and select your list provider containing the questions. then in the display part of the template place your valueholder and there should be an option to make it exactly 1, or 1 or more, or  zero or more, you pick what you want and it should allow users to pick more

then you do the same in the srl. make sure you put a loop around the value holder in the display and srl .