The scenario is the Scenario ID on the Startup page. Two official examples:
{ECC61978EDCC2B5A}Missions/23_Campaign.conf: Conflict – Everon (default){59AD59368755F41A}Missions/21_GM_Eden.conf: Game Master – Everon
The server prints every available scenario ID, including those from your mods, to the console on startup (it runs with -listScenarios). Copy one from there, paste it into Scenario ID, and restart.
