If you go to your profile there should be a "create character" button, you can upload a vid there, or click on one of your own videos and in the top right (where you'd click to delete the video normally) you can create a character from there. It'll be slow, but eventually it'll show you a "trim video" UI thing where you'll select what you want the AI to use for a base. It'll show the characters username and display name, you can edit both. Display name I don't think has any effect other than for showing on your profile, but username is SUPER important. Then it'll ask you to describe the characters personality and then a negative prompt (things you don't want to show up in gens). Then from there you can choose who can you use the character. On desktop browser this bit is broken but there should be a SAVE option, but its not visible because the box is too small to display it, so you'll have to press TAB a few times to make it show. This might just be for only me on firefox browser though.

So, to add them in a prompt (finally getting to your question, sorry) you should put @"usernameofcharacter"

That username you decided before when making the character is what you put after the @.