Log in to your Jira Server instance.
Browse
<Base <Jira_Base_URL>/rest/api/latest/user/search?startAt=0&maxResults=1000&username=<Enter_username>
You will get the userkey in the output.
...
Log in to your Jira Server instance.
Browse <Base <Jira_Base_URL>/rest/api/latest/user/search?startAt=0&maxResults=1000&username=<Enter_username>
You will get the userkey in the output.
...