Buildpack will be supported by MindSphere as of tomorrow 20.02.2019.
There is a demo application – https://appstore.home.mendix.com/link/app/108810/
Using the following connector it is supposed to work - https://appstore.home.mendix.com/link/app/108806/
Here is the How-To – https://docs.mendix.com/developerportal/deploy/deploying-to-mindsphere
What does a webkey consist of? If it is only a token, have you tried entering it at the password field and leaving username blank?
You might be the first person trying to deploy to the Mindsphere version of CloudFoundry so perhaps we will still need to build support for that...
Is there any progress on this? This is a very important issue!