To make a launcher for your panel, you can either make a desktop launcher and drag it to your panel (where a copy of it will appear), or create it with a built-in feature of the panel as follows.
Right-click an empty area of your panel, and choose Add to Panel. At the top of the app that appears you will see Custom Application Launcher, so select it then click +Add.
You will see firstly there is the Type; just leave it as Application. For Name, put something descriptive that will be the tooltip that will be displayed when you hover your mouse over it. For an example, we’ll use the eject command, so for Name put something like Eject Disc. For Command, simply put eject, and for Comment, something like Eject the DVD Tray.
Note that the command can have options after it without the need to enclose it all in quotes or anything. Also, depending on the command, you may need to change the Type to Application in Terminal if you find nothing happens when you click the launcher.
Before you click OK, you’ll see a generic icon which you can click on to select one of your own choice to replace it. It should display some basic system icons to choose from, but you can always Browse to a folder where you keep your favourites.