What you are looking for is "source NAT". For example, a 192.168.1.10 source address will be translated to 208.x.y.z when traversing from 'inside' to 'outside'.
This is done via "IP pools" in FortiOS. For the example, it is sufficient to tick "NAT" in the policy which allows sessions from 'inside' to 'WAN'. Then the (current) WAN interface address will be used for source NAT.
If you want to have full control over the source address then create an IP pool and specify it's name in the policy.
Please have a look at the concept in the 'FortiOS Handbook', available for download at docs.fortinet.com . You'll see it's quite easy once you get the grasp of it.
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