Depends on how much current draw you plan on using at the adapter.
The safest and simplest way is to wire a fused connector directly to the battery. Then route the wires to the location you want and mount the adapter to the body work on the scoot.
If you want the power to be switched (on by the ignition switch) Then you will need to run a power relay (amperage rating on the relay depends again on your current draw).
The relay can be tapped into the ignition switch.
There is a post in the wiki and perform a search on the forum. There are several posts on how to do this:
LX150: Installing a 12V Power Outlet
Thanks
Joe