On here,or Derpi,you write down the tag,press enter,or the little green arrow if you're on mobile. Seems simple. Unless you're typing in a tag they don't already have,then you have to switch to plain editor and hope what you just typed didn't disappear.
Character names are especially a bitch. If it's a new name or one they haven't classified as a name yet,they want you to request on discord to classify it.
Maybe it's just because I'm on mobile it doesn't work as simply as it would if I was just using a PC.