You need a riseup.net account. You need to know your riseup account name. Go to https://user.riseup.net/users/riseupusername/vpn to obtain your VPN secret. (VPN password) (Replace "riseupusername" with your actual riseup user name.) (Or just got to https://user.riseup.net, login and click on "VPN".)
Open /etc/openvpn/auth.txt in an editor with root rights.
Add. (Add your actual user name and password.)