I think that the right way of using the plugin should be "after onboarding" and plugin should not call onboarding API. The flow should be something like this: 

1. User registers to use QtCloud
2. On Getting started page, show a link "Configure your Gitlab project to use QtCloud CI"
3. On "Configure Gitlab project page", show the link to download the plugin
   1. The plugin can manage the communication between Gitlab and developer computer, plus implement some "advance features" with Git which otherwise are bit complicated with command line, especially - git push with options and managing builds with specific environment variables.