How do I manually configure the Link module?
You can override that in a container services file.

For example, create/edit /sites/default/services.yml, and add/alter the following lines:

parameters:
  filter_protocols:
    - http
    - https
    - ftp
    - news
    - nntp
    - tel
    - telnet
    - mailto
    - irc
    - ssh
    - sftp
    - webcal
    - rtsp
    - file


and make sure you have this line uncommented/added to settings.php:

$settings['container_yamls'][] = $app_root . '/' . $site_path . '/services.yml';