I think what most people do is either make a thread with the resourcepack link pointing to PlanetMinecraft, and then use the Open Book player action with the full thread URL within the book, or if you are not an emperor or above, you can just use "SendMessage" (example:
https://mcdiamondfire.com/threads/test-thread.1). You may be able to use a planetminecraft URL too without a thread redirection
I don't recall if you still need approval for usage of resource pack distribution, I don't know if that guideline has been modified, so you might want to check with a moderator that's online just to be sure. But I hope this helps!