Hi,
In VPS domain forwarding, source IP is the IP of the server on which VPS is hosted. By default primary IP of the server is considered as Source IP.
You can also set the source IPs to use under Admin panel > Configuration > Slave Settings > HAProxy settings
Note, for Domain forwarding to work, it must be already enabled under Admin panel > Configuration > Master Settings > HAProxy settings
It you still found the provided information insufficient, then provide us more details regarding what exact steps you are following.