Hi,
If you just need the feature and don’t intend to get involved in development, I would suggest to just wait. It shouldn’t be long now until 7.0 is officially released.
You can’t “install a patch file”, you need to build a binary from the code where the patch has been applied. The workflow would be to get the source code from git (since the patch probably doesn’t apply cleanly to the downloadable 6.9 source archive) , apply the patch, and build. This isn’t trivial if you don’t have experience with this kind of things (but if you’re interested you could of course have a look at Download Ardour | Ardour Community).
Here you don’t have to get the patch, since it has already been merged in Ardour codebase. If you want to test ahead of release, you need to build the -rc1 version from git (see the link above and http://ardour.org/development.html) or use a nightly build from https://nightly.ardour.org/. Be aware that there is no warranty or support on those builds, and you need a subscription to get a full-featured version.