iPhone/iPad: in Safari open https://onekitplus.com/app/?standalone=1, tap Share, then "Add to Home Screen".
Android: open https://onekitplus.com/app/?standalone=1, then use the menu (3 dots) and choose "Install app" or "Add to Home screen".
Desktop: open https://onekitplus.com/app/?standalone=1 in Chrome/Edge, then use the install icon in the address bar (or Menu -> Install).