Add Custom field value through API or Webhook

[Nodar Vardiashvili]

Hi,

We are using a soft Dynamics NAV (formerly known as Navision). There we have a user's list, who is the manager and who is subordinate to whom. 

I want when the user creates a ticket in Jira, the Approver field must fill from Navision's webhook or API key. Is this possible? I'm trying to do this from the post function, any ideas?