To add moderators to your server instance you have to edit(.local/shared/American Truck Simulator/server_config.sii) example seen below:
moderator_list: 3
moderator_list[0]: Steam64_ID
moderator_list[1]: Steam64_ID
moderator_list[2]: Steam64_ID
^^^ Steam64_ID can be found by googling Steam ID Lookup
A steam server Token is required
Adding a server token received from registering your server at https://steamcommunity.com/dev/managegameservers be sure to use the correct App ID or your server will crash upon startup.
American Truck Sim = 270880