Adding a new dependency to a module
@Ivan Jaros is correct.


a) use dependency in your module info file, b) implement hook_requirements, c) activate the field only if the other module exists(ie. soft dependency).


  @Kevin, after you've enabled the module


Use a) and c) above.


  
  Drupal end-users get prompted to download and install the new dependency.
  


In c) you need to set a message to the user explaining why the field is unavailable and where to get the dependency.  Use function drupal_set_message.

Links:

D8: module exits, https://www.drupal.org/node/1894902 , see \Drupal::moduleHandler()->moduleExists($module)

D8: set message,  https://api.drupal.org/api/drupal/core%21includes%21bootstrap.inc/function/drupal_set_message/8.4.x