Unifi UDM Pro - Site to Site IPSEC VPN with SNAT

I need assistance with creating an IPSec VPN Connection from a Unifi UDM Pro router/firewall to a 3rd party / vendor. I have all the required information regarding IPs, IKE info, shared secret etc, but lack the knowledge to configure the SNAT part of the configuration since it can't be done from the UI and needs commandline experience with the UDM device.

I need someone with technical skills and experience with Unifi VPN and/or linux IPtables experience to asisist in creating the connection.