How to display entity label using entity reference field checkbox form display
I assume the context_question is your custom entity or coming from a contrib module?

There are two options:


Add label key to context_question entity definition under entity_keys so that Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection pick it up. See Aggregator item for reference.
Add a @EntityReferenceSelection plugin. You have to define one. Check TermSelection for how it is implemented for terms. It looks like this: