Sub WriterOptionSet( _
   ByVal _eType As eMFWriterOption, _
   ByVal _bsName As String, _
   ByRef _ppProps As IMFProps _
) 

void WriterOptionSet( 
   eMFWriterOption _eType,
   string _bsName,
   out IMFProps _ppProps
)

procedure WriterOptionSet( 
    _eType: eMFWriterOption;
    _bsName: String;
   Out  _ppProps: IMFProps
); 

void WriterOptionSet( 
   eMFWriterOption _eType,
   BSTR* _bsName,
   [PARAMFLAG::Out] IMFProps* _ppProps
) 

Parameters

_eType

option's type (format, video or audio codec)

_bsName

option's name (full or short name)

_ppProps

properties of the option (attributes for configuration)

Here is example of how set video codec by its name:

IMFProps props;
m_objWriter.WriterOptionSet(eMFWriterOption.eMFWO_VideoCodec, "mp4", out props);

Reference

IMFWriter Interface
IMFWriter Members