In order to edit your server's command line for your Garry's Mod server, you'll need to follow these steps.

Step 1: Log into your Host Havoc control panel (https://gamepanel.hosthavoc.com).

Step 2: View your game service.

Step 3: Click on the 'Commandline Manager' module.

Step 4: Head over the 'Custom Commandlines' tab.

Step 5: Create your new Commandline.

Step 6: Click the 'Select' button.

Step 7: You're all set!