When I run some apps via PlayOnLinux, they are shown in Plank, but there is no way to pin them. How can I do that anyway?
You might need to add a .desktop file to ~/.local/share/applications
I would check to see if there is one in /usr/share/applications. If there is, copy that to the first directory I listed. If one does not exists, you might have to create one manually. But if you can launch it from the wingpanel than you have a valid .desktop file. It just need to be in the right place in order to pin it.