I want the form to sit outside of all the sites, read a site and a list to write to

I am trying to have a to build a Request for Information form sitting outside of the sites (Project sites). The user should be able to start the form, and select which site they want to write to ("Which project are you part of?"). Ideally all sites will have a "RFI Register" list that this will write to. 

I don't know where to start. Any guidance is greatly appreciated.