Normally this value stays as-is. If you are using GitLab Enterprise then you should enter the URI to the API root of your GitLab installation (e.g. https://gitlab.company.com. The plugin will automatically add "api/v3" suffix for you.).

Notes:

  1. The https:// / http:// part needs to be specified.
  2. There should not be any trailing slash (/)
  3. For GitLab Enterprise, the API URI is usually the same as the Web URI.