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