Programmatically load 'add content' form for custom content type in form or block plugin
Entity forms are built by EntityFormBuilder::getForm(). The parameter is an entity not a form class. This can be an existing entity or a new entity to add content.

For example to add content for the content type article you create this entity:

$node = \Drupal\node\Entity\Node::create(['type' =&gt; 'article']);


Use the new entity to build the form:

$form = \Drupal::service('entity.form_builder')-&gt;getForm($node);