This requirement is generally handled by a network security Engineer.
As per your post I understand that your internal hosted server wants to communicate to a remote server hosted in another network right?
Your internal hosted server should source NAT to an available static public IP allocated by ISP and we need to frame an outbound security policy in firewall as:
Source interface : inside interface of firewall
Destination interface : outside interface of firewall
Source address : server private IP(local IP)
Destination : remote server public IP
Action : allow
Security profiles :on
At the same time the server source NAT IP needs to be whitelisted at teh remote client, so that our traffic is allowed through the vendor firewall
Let's see the traffic flow
When local server initiates traffic
- source IP is local server private IP
- destination IP is remote server public IP
- port tcp-1234
When traffic reaches the local hosted servers network perimeter firewall, the server local private IP is translated with the sourced NAT'd public IP
The source will be a local server source NAT'd public IP
and destination will be remote server public IP with port tcp-1234
Now traffic will reach to the vendor. The vendor has already whitelisted our source NAT'd IP so traffic is allowed and will access the remote server on port tcp-1234
In the same way, reverse traffic flows and our local server will be able to access resources on the remote server.