This app will allow you to put a button on the log in page that displays a text and a link of your choosing. Customize the link by adding the following options to your config.php, e.g:
'login_page_button_text' => "SUNET",
'login_page_button_link' => "https://sunet.se",
To install simply place this app in nextcloud/apps/