"Pageant" is installed in the PuTTY installation folder along with PuTTY itself.
Double-click "pageant.exe" to start it and make it resident in the task tray (window does not appear).
Right-click on the Pageant icon in the task tray and select "Add key" from the menu that appears.
In the dialog that appears, select the private key (PuTTY format .ppk file) for logging in to the login node. You will be asked for a passphrase, then enter it.
This completes the registration of the private key. By doing this, you can omit inputting the passphrase of the private key when logging in to the login node with PuTTY.