Go to Jira project configuration ** In Workflows tab Click EDIT Icon.

In Post Functions tab section, Click Add Post Function.

Choose HTTP Request API Invoke Function and Click Add.

Configure Post Function and Click Add.

Here is example to use Issue fields in JSON payload:

{
 "tracking": "{issuekey}",
 "team": "{customfield_10201}",
 "customer": "{customfield_10200}",
 "release": "{fixversions}",
 "owner": "{assignee}"
}

Note. You can find custom field ID at, Issues > Custom Fields > Click Edit > You will find in URL.

summary, description, key or issuekey, type or issuetype, status, priority,
components, labels, fixversions, affectsversions, assignee,
reporter, environment

To generate basic access token with username and password:

$ python
>>> string = 'username:password'
>>> data = base64.b64encode(string.encode())
>>> print(data)
b'ZGF0YSB0byBiZSBlbmNvZGVk'
>>>
//your basic access token: ZGF0YSB0byBiZSBlbmNvZGVk

Review and Click Publish.

Transition Issue and verify if HTTP Request triggered with response notification in issue comment.