To pin the FTB app onto your taskbar you need to pin it from within the actual application folder as opposed to trying to pin an open instance of the FTB App. In order to do this first open your start menu and search for FTBApp. This should show you the FTB App under best match. You can then right click on this icon and select pin to taskbar.
On MacOS, use spotlight to find the app by searching for FTB App, once found, open the App. You should now have an open FTB App in your Dock. To keep this here, you'll want to right click on the FTB App icon in the dock, hover over options, and press Keep in Dock.
Linux has many desktops and if you're using one other than Gnome, you'll need to google it 😃. If you're using Gnome, you can add the app to your dock / taskbar by first, opening the FTB App, then right clicking on the App in the taskbar / dock and click Add to favorites.