Ah, a more interesting problem. I like those.
I think you'll need to do something with CL_HTTP_CLIENT - maybe something like this?
DATA client TYPE REF TO cl_http_client.
CREATE OBJECT client.
client->request->set_header_field(
NAME='SOAPAction'
VALUE='https://evatr.bff-online.de/' ).
Then use various method calls to send parameters and retrieve results. I've not had a chance to test it at all. I think maybe this will give you some hints. (Also in German).