Arena should allow URLs to open the client and prefill a Direct Challenge with format options, so that players can just pick their deck and go. This would allow Discord bots and tournament organizers to provide one-click match start solutions. The URL can be a HTTP URL that goes to a page that has an Open Arena button or auto-opens Arena; some OSes, like iOS, macOS and Android, can also intercept the URL and open Arena directly

For example, a URL could be something like this:

http://arena.wizards.com/challenge?player1=millenomi%2395099&player2=example%2301234&format=historic&tournament=true

Starts a Direct Challenge between millenomi#90599 and example#01234 with Historic decks and Tournament Challenge mode enabled.

http://arena.wizards.com/challenge?player1=millenomi%2395099&player2=example%2301234&format=standard&bestof=1&start=cointoss

Starts a Direct Challenge between millenomi#90599 and example#01234 to play Standard Best of One. The player who begins is chosen by coin toss.