Drupal 8 has included an Entity Autocomplete element, which can be extremely
useful when referencing entities within a form.
The basic render array syntax is as follows:
This will simply display a text field which will autocomplete after a certain
amount of characters are entered.
You can target different entities with
#target_type, like so:
You can target different bundles with
target_bundles, like so:
To display the default value, you will need to assign the loaded entity:
You can find more information on Drupal.org.