Upload the mission file
For uploading your own mission file you have to connect to your server via FTP before: Gameserver FTP connection. There you have to upload the mission file into your mpmissions directory which you can find under /gxxxxxx/arma3/. In this example we upload the mission file King_of_the_hill.Altis to our server.
Loading the mission file on the server
To load the mission file from the server, go to your ArmA 3 server interface and click on the Configs tab in the menu on the left:
There you edit the file server.cfg and enter the name of your mission file as shown in the picture:
After that, click Save and you can start your server. The new mission file will be loaded.