I hope you are using request API to make API calls to your third-party API’s.
There is a feature called static IP in request API with all IP ranges, you can make use of this feature to whitelist it in Jira.
Here I need to access the Jira APIs not the Freshservice APIs.
So, when a request is sent from the Freshservice custom APP (via a particular IP address), first this particular IP has to be whitelisted in the customer’s Jira account, then only we can access their URL and the respective Jira APIs.
Please let me know if this clarifies all your doubts otherwise let me know if we can have a quick call to discuss further.
As I said, you can use Request API with Static IP true and if you enabled this feature, the Request going from the App will always be in this IP ranges and you can white list this IP in the customers Jira account.