WCF / ASMX :: Dialog Box Should Be Popup While Downloading/saving The File Locally?

I have a WCF server for downloading the file from server to user's local drive, when user clicks the download button on client.aspx page.

Currently i am giving hard-coded path for saving the file, but need to open Dialog box to ask user's to get location to save the file on drive.

My Client Code is:

//Service1Client client = new Service1Client();
client.Open();
string s = client.GetData(5);
stream1 = client.GetFileStream("20101102.zip");
string filePath=@"c:Test";
outstream = File.Open(filePath, FileMode.Create, FileAccess.Write);
//CopyStream(stream1, outstream);
const int bufferLen = 10000000;
byte[] buffer = new byte[bufferLen];
int count = 0;
int bytecount = 0;
while ((count = stream1.Read(buffer, 0, bufferLen)) > 0)
{
outstream.Write(buffer, 0, count);
bytecount += count;
}
}