The default wordpress add post page is very clean with only minimum of form display. The visibility of custom fields on the post edit screen is controlled on the post edit page. There’s a button called “Screen Options” towards the top of the screen.
Clicking that button will display all of the available boxes and panels for the edit page. If custom fields is not visible here, then either your theme or a plug-in is disabling the feature.
If everything ok, than you should see the new added custom field form appear at the page of posting as per following.