post
 

Could I use onto and in? (not in and on)

-They will have to post __ the assigned area __ Greenboard something.

Greenboard is the name of an online system where you will be able to find information about courses that you have registered for. Normally I would say onto in collocation with area is wrong and in should be used, but I guess maybe it depends on what that 'something' is and onto could be correct with area.

Re: post
 

No - 1) in 2) on

Something is located in an area and on a website-
So :

They will have to post in the assigned area on Greenboard.

If it were page rather than area then both prepositions would be on :

They will have to post on the relevant page on Greenboard.