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:
-crossplay
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 the "Custom Commandlines" tab
Step 4: Click New.
Step 5: Scroll down and locate the "Additional Parameters" section
Step 6: Enter -crossplay in the Additional Parameters text field.
Step 7: Save the CommandLine.
Step 8: Click Select on the right-hand side of the page.
Once you have clicked Select, your service will restart. Instructions on joining your service can be found here