The form URL used on registration has nothing to do with the URL used for display the registration form. So the only way for CB Auto Actions to know where they're coming from like that is to have a form input that provides that information. This means you'd need to have a hidden input in the form where you set its value based off the URL they're using to view the registration form. Easiest way to do that is probably something like the below.

Global
Triggers: onBeforeRegisterFormDisplay
Type: Code
User: Automatic
Access: Everybody
Conditions
1: [get_itemid] Equal To MENU_ID_HERE
Action
Method: HTML
Code:Output
Display: return

Note this is untested and just an example. You very likely will need to make adjustments. Your redirect auto action acting on onAfterSaveUserRegistration should in theory then be able to condition against [post_HIDDEN_INPUT_NAME].

Looks like that trigger adds its results outside the form element. The alternative then is to use a Custom HTML field and add your hidden input there. You'd then condition the Custom HTML field to show/hide using CB Conditional and custom value conditioning. Example as follows.

Field: Custom > Value
Custom Value: [get_itemid]
Translate Custom Value: No
Operator: Equal To
Value: MENU_ID_HERE
Translate Value: No