You can pick whatever names you want. But it is much easier to use descriptive names. Now it is time to add the link button and respective group. Since this is pretty straight forward I didn't include any labels. However there is something different with this button.
Buttons have the property of being enabled or disabled.
Enabled is just the property to turn on or turn off the button. The default is on, but I changed it to off. I want you to do pick the object and vertex before you link anything. That way you don't have problems. The off is a built in value, like true, false, on, and undefined.