How to make Network manager run macchanger

I run the latest stable Trisquel. And in Network manager I have the option to change the MAC to whatever string I choose.

Also, do I have to connect to that network blindly before having the editable entry?

When I connect the GSM phone by tethering I get a new entry in the Network Manager I think every day. So my tweaks are lost every once in a while.

That's all about Network Manager for now.