Inject a shopify page through shopify-api in Rails - ruby-on-rails

Question

Im trying to inject a new page to a shopify-store through the shopify-api which is included in the shopify-app gem. I added the authentication from the gem to my controller, which works perfect for GET-requests. Somehow the POST-Method does not work.
The method inject_page_method shall be called in my create method, which would then represent a second POST request.
Im confused, because the GET-methods already work. So I thought, the gem routes this endpoints for me like this:
def inject_page_method
#create_new_page = ShopifyAPI::Page.new(:title => "test", :body_html =>"<h1>Test</h1>")
#create_new_page.save
end
helper_method :inject_page_method
Finding out this does not work I tried it with a full endpoint request:
def create_new_page
page_data = {
"page": {
"title": "Test",
"body_html": "<h1>Test succeded</h1>"
}
}
#json_request_url = #shop_domain + "/admin/api/2020-07/pages.json"
Rails.ajax({
type:"POST",
url:#json_request_url,
data:page_data
})
end
Heroku logs --tail shows me, that the method is POST, which is fine, but I get a status=500 with following errors:
NoMethodError (undefined method `+' for nil:NilClass):
app/controllers/my_controller.rb:101:in `create_new_page'
app/controllers/my_controller.rb:37:in `create'
When I change the concatenation of #json_request_url to:
#json_request_url = "#{#shop_domain}/admin/api/2020-07/pages.json"
I get a status 500:
NoMethodError (undefined method `ajax' for Rails:Module):
Which is impossible because both Jquery and Rails/ujs are required in the application.js
Edit: This is on rails 6
What is missing?