Add an integrated zimlet in the New dropdown list menu

Hello there

I’m trying to add a new Integrated Zimlet in the New Message dropdown list menu. When I do a click in the arrow I see options like Message, Contact, Contact Group, Appointment, task, etc. and I need to add a new element under the Message Item.
I can’t figure it out how to do this since I’m new Zimlet developer and I don’t know how to access this toolbar…
I will appreciate some help.
Thanks