401 for mattermost slash command intergration

I configured a slash command to POST to https://myserver.mydomain.com:12345/foo. This REST end point requires GSSAPI authentication (token authentication). The user that runs Mattermost already has the right authentication in its environment variable, however, when Mattermost makes the POST request, the env variable is not included in the header.

How do I configure Mattermost to send this extra token header when making slash command requests?