There is no linking necessary, other than joinining the "November Joy Rides" Challenge on the Challenges page, and then recording the rides by a Garmin device (imported and manually created Activities do not count). However that badge is awarded only after 75km (46.6 miles).
If you meant another badge, you'll tell us which one exactly you are concerned about. This is the only active Badge Challenge for cycling at the moment though. Other than that there is the Indoor 20-Mile Ride badge, and the 20-Mile Ride that you could be referring to. No challenge to sign necessary for either of them. For the first one you just need to record an Indoor Cycling type of activity with a Garmin device, and for the second one you have to record any outdoor type of cycling activity with a Garmin device and it must be using GPS for recording the activity.
No need to sign up for it or "linking" it. If you do what you are required to do you will get the badge.
The requirements are:
Country: UK (to see that you got the correct country setting check if you see the badge as available. Now is too late but next time check in available badges). Now you login here https://www.garmin.com/account/profile and check the location setting.
Distance: 22 miles or 35.41 km. Make sure that you really biked the correct distance and a little longer. If you are a meter/yard short you will not get the badge. The interface is rounding the value so if you just biked the exact distance you might have missed it by a tiny bit even if it says you biked 22 miles. I did 36 km just in case. Never ever just do the exact distance.
Device: Always use a Garmin device. It might work Rouvy or Zwift but not guarantees. Manual activities will not work at all. I always log on both Rouvy/Zwift and my Edge 1030 and merge the FIT files just in case.
Activity type: Cycling or any sub type. I saw that someone posted info that you could ride indoors even for the "Out" badges but no guarantees to that. You could test and change the activity type to Road cycling and see if that works.
There might be some confision here as it was a charity ride. 'Signing up for the ride' is not a pre-requisite, and has nothing to do with getting the badge. For the badge you just need to do the activity (ie a 22+mile bike ride) and upload to Garmin Connect. I got it by syncing after a MTB ride (Garmin watch + Connect) and it automatically appeared.
