Drupal 8 Webform: Prepopulate entity reference (select, radio, ...) with current entitiy / node ID via tokens

Several times in the last months we had the requirement to prepopulate a Drupal 8 webform with a parent entity ID from the URL. For example we had forms to register for events which we embedded on the node page as paragraphs or blocks and wanted to set an entity reference field to the node ID programmatically.