Epiconnn While not being able to open it in the game, if you're running it in a web browser you can add a Browser object and can do the following:
Browser -> Page is visible
Keyboard -> On G pressed (an example key)
Then add Browser -> Open URL in new window -> and enter the URL of the forum you want and the title of that window. Now, whenever the game tab is active and visible, if you hit G it'll open up a new tab or window with your forum topic in it.