Idoc to file problem with DynamicConfiguration

HI SAP Guru's

I have a iDoc to File scenario and the output file requirement is bit tricky.

output file should be xx_Region_iDoc Number_YYYYMMDDHHMMSS

I been through couple of blogs but non of them mentioned about the iDoc all of them showed the about the FileName in FileAdapter.

I believe it is only possibly via Dynamic configuration.

I need to extract E1EDKA1/LAND1, DOCNUM from Source IDoc Structure and then concat it with Date and generate the output file name.

I cannot even get the iDoc Number i.e. DOCNUM via Dynamic Configuration.

I was trying this method but it didn't work.

DynamicConfiguration conf = (DynamicConfiguration) container.getTransformationParameters().get(StreamTransformationConstants.DYNAMIC_CONFIGURATION);

DynamicConfigurationKey key = DynamicConfigurationKey.create("http://sap.com/xi/XI/System/IDoc","DOCNUM");

String ourSourceFileName = conf.get(key);

return  ourSourceFileName; 

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Can somebody please let me know how to achieve this and how?