You can do it through ZMI, accessing portal_controlpanel tool.
At the bottom you have a form for adding new items:
Name: user friendly name
Id: choose one, unique and with no spaces or other special chars
Action: is your URL. If you need an expernal link type string:http://something.com/
Category: this controls if you link will be in the Plone native section above or in the add-ons section below
Visible: put true
There's other options, but they are only needed if you need some special security check or want an icon (copy from other already defined actions in that case).
You can do it through ZMI, accessing
portal_controlpaneltool.At the bottom you have a form for adding new items:
string:http://something.com/There's other options, but they are only needed if you need some special security check or want an icon (copy from other already defined actions in that case).