Daniel Stone authored Jul 03, 2017 and Ana Rute Mendes committed Jun 22, 2020

Mattermost OAuth requires two separate URIs, whereas the Phabricator
OAuth server only allows returning to a single URI. Special case
Mattermost and get on with life.

The correct fix is to actually allow multiple values in the OAuth
configuration. I don't know off the top of my head how this would work,
e.g. a tokenising field, or a multi-line field (which I don't know how
to achieve without Remarkup), or ... ?