You can specify an ingress region (by allocating a region-specific IP), and the egress region is likely to be the same: Multi-region application where users / sessions are pinned to one region? - #5 by ignoramous
But otherwise, it isn’t possible to assign outgoing IPs as of today: Set/get outgoing IP address for whitelisting on external APIs/Hosts - #12 by ignoramous | Outbound IP from a certain region - #2 by ignoramous