PC Steam
Ok, so first, you should navigate to the Events & Custom Matches menu.
Then, click on the Create Game button
After that, click on the Mods button in the menu.
Paste the URL that you have copied with the green button from the mod post into the textbox there, and click download.
Finally, select the mod that appears in the list and click the apply button. You can create the game after you have tweaked the settings (such as server region and played limit).