Select "GameUserSettings.ini" from the drop-down list and click the "Submit" button.
Under the "[ServerSettings]" category at the top, create a new line starting with "MapModID=".
Add your mod IDs after the equals sign, separating multiple IDs with commas.
ARK mods can be obtained from the Valve Workshop. Your mod ID can be found by looking at the URL of the mod you have selected. Specifically, look in the URL for "id=". The number immediately following that are your mod IDs.