To cook an item you need to create a cooking recipe, and you'll need some sort of fuel as well, which you need to register as a fuel.

minetest.register_craft({
   type = 'cooking',
   recipe = 'group:tree',
   output = 'candy:syrup',
})

minetest.register_craft({
   type = 'fuel',
   recipe = 'group:tree',
   burntime = 10,
})