On the web version (as in mobile), assuming that users can't search/ask questions without selecting an Assistant, I've hidden the submit icon and show it only after an assistant is selected.
On desktop, after opening (or typing @username) the assistant's menu, you can select them with arrow keys (UP & DOWN) and hit Enter.
Added that X (close) icon on desktop when users hover the Assistant added. May be changed with a switch indicator or something else.
Room for improvement:
Not happy with how I handled the textbox filed. Will need to restructure it a bit to make it more responsive on desktop.
After manually opening the @ menu and selecting an assistant, should auto-focus on the textarea so users can start typing the question immediately.