Help with Drupal 9 module

Fix the routing.yml file in my module.

I have a small module that should allow me to pass a parameter to the 'node create' form

ie http://mywebsite.com/node/add/myContentType/myVar

myVar shgould then be available in the node create form for my content type so that it can be used to prepopulate one of the node fields.

I have struggled to find the correct documentation on the internet, so need someone to have a look at this and show me where I have gone wrong.

I think my problem lies in defin ing the form to call in the routing.yml file .. but I may be wrong