Using WAN as pptp

[booker]

I wanted to do something simple (I thought) …

My router’s LAN ip is 10.100.0.1/24

I want to setup WAN to connect to a pptp server with ip 10.100.20.1.
I go to PPTP configuration, i enter my username and password, i use 10.100.20.1 as “Remote IP address” and i stuck to “Local IP address”.
What should i use ???