As said above, there are a few steps to follow in order for a player to get a custom enchant on their gear. I will list them down right here:
Buy a custom enchant. Before you can apply a custom enchant, you will have to own one. Do you already own one? Skip to step 2. If you do not own a custom enchant, you will have to get yourself one. There are multiple ways how you can do this: You can buy them with in-game money from other players or you can grind enough experience and type /enchant to buy yourself one.
Once you own this certain enchant you want to apply it to your gear. It's useful to know where your enchant is applicable to. Once you've found this out, you can just simply type /anvil (or craft an anvil and use this) and put your gear in the first slot and the custom enchant in the second.
I hope that this little tutorial helped you out. If it hasn't, you can always contact me by creating a conversation with me, or by sending me a PM on discord (RawrWithBrem#6537)