I understood, I am a moron
location /forum/ { try_files $uri $uri/ /forum/index.php?$query_string; }
location /forum/api { try_files $uri $uri/ /forum/api.php?$query_string; }
location /forum/admin { try_files $uri $uri/ /forum/admin.php?$query_string; }
But my admin panel does not work for some reason
403 Forbidden
You do not have permissions to access this page.