What matters most is to convince people that planting public trees on private property behind the sidewalk lies in the common interest of the community. There is no single way that communities get this done.
Some communities use easements , treating the tree like a sort of utility pole.
Other communities fill out a very short written agreement with homeowners. Still others have no formal mechanism , they just do it and nobody complains. But in all cases, keep it simple. It takes communication, education, and negotiation between homeowners and local officials for setback planting to work. Again, arrangements vary from community to community.