It should be a permanent link. Is it throwing an error?
The link seems to be functional, could you post a screenshot of the error?
Also -- how many Discord servers are you in? I used to have a similar issue as I was in too many servers at once. Discord sets a hard cap on how many servers one account can be in. If you're at the limit you may need to leave another server before you can join a new one.
4 days ago(+1)
Weird... the link seems to be working fine for others.
In the meantime, maybe try this link? I have it set to expire in 7 days, as a heads up.
If that doesn't work -- try going to the "Discover" section of Discord (bottom left, next to your account), and search up "Boss Rush Jam".