For this step, a riseup.net account and Riseup account name is required. Go to https://user.riseup.net/users/riseupusername/vpn to obtain a VPN secret (VPN password). Below, replace "riseupusername" with the actual riseup user name, or just go to https://user.riseup.net, login and click on "VPN".
Open /etc/openvpn/auth.txt in an editor with root rights.
Add the actual user name and password.
Save and exit.