Never used r2modman, but all you're doing is adding an 'argument' to the execution of a program. Some launchers may have a separate field for program arguments, to which you would just add
others might just have you suffix it to the program name. Same as if you were running it from a terminal.

Linux:
Windows:

Server package includes a bash script / batch file (depending on OS) which you can edit to as needed, or if you like typing, you could run it from a terminal like this

If your using Bepinex (assuming you're modding), the package includes modified bash scripts for the game client and server, which you should use, editing as needed.

The console is a GUI client function, and not included in the argument list when launching a server. In order to admin the server from the client console, you'll need to add yourself to the adminlist.txt. Plenty of guides out there for that.

Be advised that "devcommands" / cheats cannot be used by a client connected to a dedicated server, even if your listed as a server admin and have the console enabled for your game client.