I’m not sure I quite understand what you’re trying to achieve with the%u substitution. Are you trying to use the ThinLinc client to open a local file, remotely?
The behaviour you’re seeing in the screenshot is because there is no window manager being started with the application. Using tl-single-app will ensure that a minimal window manager is started along with the application itself, and things should work a lot better. More info here:
Another option would be to create a ThinLinc profile which only launches the desired application. This approach is described in the following article:
One final point, we always recommend running the latest version of the ThinLinc client and server (at time of writing, 4.19.0). So if you have the possibility to upgrade, it would be a good idea to do this first.