CryptMsgOpenToEncode

[DllImport("Crypt32.dll", SetLastError = true)]
public static extern IntPtr CryptMsgOpenToEncode(
   uint dwMsgEncodingType,
   uint dwFlags,
   uint dwMsgType,
   IntPtr pvMsgEncodeInfo,
   IntPtr pszInnerContentObjID,
   IntPtr pStreamInfo
);

Microsoft documentation: Link