If you are using OpenVPN in a Linux platform through command line, it is always handy to be able to save the authentication information in a file and let openvpn use them. There are two ways you can do it.
First Method
First save the password in a file e.g auth.txt with two lines:
username
password
First line is for username and the second line is for password.
If you are using .ovpn files for configuration, open the .ovpn file and simply add the following:
auth-user-pass auth.txt
Now, your authentication would use the authentication given in that file
Second Method
You may add the auth-user-pass in the openvpn command line argument, but you have to make sure, this is passed after the --config. Here is an example
openvpn --config "your_file.ovpn" --auth-user-pass "auth.txt"
That should be enough.