To enable crossplay on your ARK server to allow both Steam and Epic Game Store clients to connect you will need to add the following to your command line's 'Additional Parameters' option:
If you have not configured a command line, please follow these steps:
Step 1: Log into our game control panel (https://gamepanel.hosthavoc.com) and access your game server.
Step 2: Click the Command Line Manager button.
Step 3: Click New.
Step 4: Enter -crossplay in the Additional Parameters text field.
Step 5: Save the Command Line.
Step 6: Click Select on the right-hand side of the page.
Step 7: Restart the server for the changes to take effect.