First, as said, please note that 1.12 is unsupported on these forums; any threads you make about unsupported versions will be locked by a moderator (in fact, I wouldn't be surprised if this thread gets locked). You won't receive any help also from the Discord or the subreddit, as the Forge team heavily discourages people from asking help for these versions. You'll have to learn on your own, until you update.
Second, although you may not have malicious intent, the Forge team is heavily against hacked clients, and custom clients in general. You may only do these for your own learning purposes, but do not seek help from the forums/discord/reddit/anywhere the light of LexManos touches if you plan to make mods emulating client-side hacks. I strongly suggest updating and learning mods by making simple mods.
Third, the Forge API is primarily for use by mod developers to modify both sides of Minecraft, the client and the server. It is possible for create a server-only or client-only mod, but there are many pitfalls that you may fall into because Forge expects most mods to be for both sides (and most documentation and tutorials assume so), and your code may be incompatible in some way.
If you wish to receive any support, please update to a modern version of Minecraft.