Steam Link now supports Quest Pro's new tongue tracking feature in VRChat.
The Steam Link app for Quest has supported Quest Pro's face and eye tracking capabilities since launch and passed it though to VRChat on your PC, but Quest Pro's face tracking didn't include your tongue until an SDK update last week.
Valve has integrated that new SDK version in the latest version of Steam Link and SteamVR, and implemented the tongue tracking.
That means you can simply run Steam Link on your Quest Pro, launch VRChat in the Steam interface, and as long as you have OSC enabled and a supported avatar, you'll be able to stick out your tongue - no extra software or setup is required.
Steam Link also uses the Quest Pro's eye tracking for foveated encoding, meaning it will prioritize video streaming encoding to where you're currently looking.
Update December 21: this beta feature has now been rolled out to the stable SteamVR branch. This article has been updated to reflect this.