WPMU Dashboard

I've installed the WPMU dashboard plugin
The plugin asks me to enter an username, password and email to get a free api key.

I don't want that, want to insert my api-key.
Like these instructions https://premium.wpmudev.org/project/wpmu-dev-dashboard/

When i insert my account, the plugin reports an error like that this name is taken or unavailable. So that is not a solution either.

Please can you help me how to fix this in an easypeasy way? I must be missing something.

By the way, sorry to post this in 'General section' But i couldn't get it trough the 'plugin' + 'other' selection.