Start by opening the config file that you want to save the username & password for.
I have used WordPad to edit the files in this how-to.
You can find the config files in the following folder: C:\Program Files\OpenVPN\config\
Save the file as password.txt in your OpenVPN config dir (C:\Program Files\OpenVPN\config\).
You can now connect to the VPN without entering the username & password each time you need to connect.