Leave this blank unless you are having issues with webhook events failing to trigger.
In some rare cases, the URL that Gitea uses in webhook events may not be the same as the URL that Jenkins uses to access Gitea. For example, if Jenkins needs to access the Gitea web interface through a reverse proxy but regular users access Gitea directly.
If this is the case, then provide the Server URL that Gitea publishes in the webhook events. To determine this URL you can inspect an event in the Gitea Web UI. The event will look something like:
{
  ...
  "repository":{
    ...
    "full_name": "example/reponame",
    ...
    "html_url": "http://git.example.org:3000/example/reponame",
    ...
  },
  ...
}
    
You are looking for the repository/html_url value and you should remove the repository/full_name part at the end to determine the Alias URL to specify, i.e. using the example payload above, the alias URL would be http://git.example.org:3000/