It seems that scp does not have such options, but pscp (sftp client does have). So, I would be writing something like this to extend your initial arguments with that option based on the following man page:
QString program = "c:/temp/pscp.exe";
QStringList arguments;
arguments << "-pw" << "password" << "C:/Users/polaris8/Desktop/Test1GB.zip" << "Mrigendra@192.168.26.142:/home/";
^^^^^^^^^^^^^^^^^^^
QPointer<QProcess> myProcess;
myProcess = new QProcess;
connect(myProcess, SIGNAL(readyReadStandardOutput()), this, SLOT(readOutput()));
myProcess->start(program , arguments);
Also, I would encourage you to use QStandardPaths for a path like yours. See the documentation for details:
QStandardPaths::DesktopLocation 0 Returns the user's desktop directory.
So, you could eventually replace this string:
"C:/Users/polaris8/Desktop/Test1GB.zip"
with the following:
QStandardPaths::locate(QStandardPaths::DesktopLocation, "Test1GB.zip")
That being said, you may wish to consider using keys instead of password in the future. It would be a bit more secure, and also convenient for your application.