Cheatengine/Hexedit a piston to a absurd number, and then mount a thruster (Probably via scalables), Then, you add tanks ontop of the thruster, THEN you can add a sensor and detonator (Proximity Fusing if you want), and I think thats all.

easier than using cheat engine to modify the extension range of the piston, you can just copy one from an already existing ATGM on a vehicle or take it apart to see how it works
beware that changing the values inside the piston resets it to vanilla maximum, so if you try to change anything, you'd have to reimport the creation to grab the piston again, or use cheat engine/hexedit yourself

you also ideally want a detonator on the missile itself but NOT connected to the fuel tanks (or on the launcher vehicle if you prefer to do it safely), connected to a rod that attaches the missile itself to the vehicle, so that it doesn't start expanding by gravity when you look down