CryptMsgCountersignEncoded

[DllImport("Crypt32.dll", SetLastError = true)]
public static extern bool CryptMsgCountersignEncoded(
   IntPtr hCryptMsg,
   uint dwIndex,
   uint cCountersigners,
   IntPtr rgCountersigners,
   byte[] pbCountersignature,
   ref uint pcbCountersignature
);

Microsoft documentation: Link