Let's say I have a handout called "Fire Dragon". I also have a handout called Bestiary that has links to all the individual handouts including the Fire Dragon entry. I can right click on the Fire Dragon link in the Bestiary handout to get the in-game journal link, and use that for the link in the button. That calls/opens the in-game journal entry. This is a screenshot of the quick test I ran, and I didn't open the Fire Dragon handout from the Bestiary link, but by clicking "Show Fire Dragon" in chat. I pressed the up arrow to show you how that button was constructed and what the link should look like. Note that I am using a single left click to bring this up, so no right click or opening in a new tab. I don't play with the setting checked to pop things out in their own window, but I tested that way and it opened it up in its own window just fine as well. Right click and open in new tab does fail though, probably since then the new tab it is trying to open into isn't created or directly connected with the game instance anymore, since it is being generated by a browser function instead of called for by the vtt.