How to integrate "Datev extf-files" API in my nodejs project.

[#1]

I tried as per given in the developer documentation, but I got an error in the authorization process. If anyone knows the full process, please explain or provide any reference videos available there.

[#2]

You need to send Datev a "Servicekontakt" and I presume you need an paid account on datev developer.  

Here are some contact details for the contact: Support | DATEV Developer Portal

[#3]

I have a query while datev portal registeration, in final step it asks invitation code from invitation letter, How did I get that? Please explain this process.
Is there any other way to register Datev portal explain this also?