How to pass an extra argument to an entity form? Suppose, I've two entities:

entityA
entityB


A user creates an entityA and has the option of creating multiple types of entityB referenced from a single entityA. For example, a parent-child relationship. Parent being entityA and child being entityB.  

In the module, I want the users to select an entityA object, go to a new page page for creating entityB. So, at this point, (add-form of entityB) I should have the data of that particular entityA object that the user clicked on.

So, far I'm able to get "all" the data of entityA on the add-form of entityB, which is of no help as I'm unable to get the specific id of entityA.

The relevant code are:

routing.yml

mymodule.entityB_add:
  path: '/admin/config/mymodule/{entityA}/entityB/add'
  defaults:
    _entity_form: 'mymodule_entityA_entityB.default'
    _title: 'EntityB'
  requirements:
    _entity_create_access: 'mymodule_entityA'


When, I'm shown the form addition page of entityB (type: ../../testentitya/entityA/add), how do I pass the entityA object to the form and what will be the structure of entityB's config schema?
Have a look at https://www.drupal.org/node/2310425

This should explaine everything needed.

Here is an example how it would look like:

mymodule.entityB_add:
  path: '/admin/config/mymodule/{entityA}/{entityB}/add'
  defaults:
    _entity_form: 'mymodule_entityA_entityB.default'
    _title: 'EntityB'
  requirements:
    _entity_create_access: 'mymodule_entityA'
  options:
    parameters:
      entityA:
        type: entity:entity_type_a
      entityB:
        type: entity:entity_type_b