Document how to pre-populate `Application to Open` in `link.rst`

### Proposed change

The link generator is _awesome_, esp. with the nicely documented details on how to pre-populate various fields. But I didn't find documentation on how to pre-populate the radio button for which application to open...

Right now I don't quite know what the right approach should be so I'm filing it as an issue so we don't forget, I'm happy to revisit with a proposed solution if someone points me in the right direction, though it may be a quick-and-obvious fix to the team...