It depends on your distro. And it depends on where you want it.
I would create a new desktop launcher. (Right-click the desktop to find that option in Linux Mint.) That allows you to choose from many icons, including the standard Thunderbird icon.
After creating the desktop launcher, you should be able to drag-and-drop it to your panel, if you want.
You should also be able to use the Menu Editor of your desktop environment settings to create a new entry for Thunderbird, whether inside the Internet category, another category, or standalone on the main menu.
Right-click on the desktop "rocket" icon you have now, then choose Properties. Then click on the "rocket" and it will open up a screen with other choices. Choose the Applications entry on the left, then scroll down on the right until you find Thunderbird.
I see the drag-and-drop to the panel does not work though. Oops, sorry.