@leocg
There is already the association in Windows Registry:

@leocg
the HTML code of the PLAY NOW button

<button type="button" class="btn-start en" onclick="fnStartGame()">  Play Now  </button>

@leocg
I don't think it's the issue with protocol or file type association. On the same computer, the other web browsers can support the function of PLAY NOW button.

In addition, the PLAY NOW button is not a URL link. It's a program or script. The function of PLAY NOW button is not only calling the associated program on the local disk. It also checks the availability of the game server.

@leocg
Clicking on PLAY NOW button does not call the game launcher program as expected. With other web browsers, it will call the launcher. See the attached picture below:

The user clicks on Play Now button, but the game program cannot be launched.

This is an online game's official website. The players must launch the game through the web page. The players must sign in through the web page and click on Play Now button to call the game program inistalled on the local disk drive.

With other web browsers, the function of Play Now button of https://icarus.valofe.com website can work.
Only Opera web browser cannot support it.