Adding mods to the game at runtime is not and should not be possible. Forge is designed so that mods can, on setup, communicate with one another (one adds an item, the other can see that item and do things related to it). If you could add mods in game, you would either not allow other mods to communicate with the mod you added, or rerun the entire setup process again. There is not a good reason for any such feature to be added because in order to work properly it would basically just close and reopen minecraft.
Forge for 1.12 is in very early beta. Even if you get it working, the new recipe system is completely nonfunctional and there are probably lots more functions not even slightly existant. Setting up forge for 1.12 is not worth it at the moment, so just wait a couple weeks to start using it.