You need a client that supports it

https://emojis-iota.vercel.app/ has some custom emojis that you can add it & then you will be able to use it on supporting clients.

Afaik, Amethyst & web clients Rabbit & nostter.app support.

Also, on Nostter, there's emoji kitchen integration that lets you combine 2 emojis to create & use custom emojis.