Set Admin Password
You can view and change the admin password in the interface of your server under the tab "Settings". You can access the settings here:
Connect to the server
Now connect to your server. Open Assetto Corsa Competizione on your PC. Then go to "Multiplayer". Here you can click on "Serverlist" in the lower right corner. In the search bar you can enter the name of your server so that you don't have to search for it.
ATTENTION: It can take about 30 minutes until the server is listed.
Become an admin
If you are connected to the server, you can press ENTER to enter something in the chat. Here you can now enter "/admin ADMINPW". Now you are admin on your server!