The way this works is on Android app developers write code that tells the OS that the app can open specific links. If the link isn't there, you can't make the OS add the link.
Since Slide is open source, I checked the code and it does say it can open *.reddit.com links.
This shouldn't be the case, but do you have the official Reddit app installed? If you do, the link will show up for Slide, but it would be greyed out, so I'm assuming you don't have it installed.
Do you have some kind of firewall app running like Netguard? Make sure you're not blocking system apps, in this case "Intent Filter Verification Service".
If *.reddit.com still isn't showing up, you can try restarting your phone. That may trigger the OS to search for links again. Otherwise, uninstall then reinstall the app.
Hopefully this helps.