How to redirect to referenced entity form with Rules
You want to use Rules for this. It should already be installed since Drupal Commerce requires it. 

Create a new rule by going to Admin > Config > Workflow > Rules, and click Add rule.

A rule consists of 3 parts.

The event, the action that triggers the rule.

The event that initiates the rule would be After adding a product to the cart.

The conditions, the checks to make sure the rule applies.

Use this to find your reference field; 

Add a rule entity had field and check if the commerce_line_item (I suppose you get that by default) has a field commerce_product. Then add a second check to see if commerce_line_item:commerce_product has a field entity_reference. 

Depending on where you created the field you might require several layers.

The actions, what to do if the rule is valid.

You want to use the Page redirect action in the system group. Make sure to click the data selector button, then you can search the redirect path. It suppose should look something like commerce_line_item:commerce_product:field_reference:url.