No Callback with Authorize.net – Do I need Default receipt link url?

Issue: Once people select a membership and pay (Authorize.net), the payment goes through but they are not taken back to the site and the payment is NOT recorded and does not update their membership.

I am using Authorize.net to process memberships.
I have added the correct Login/transaction info under Payment Gateways and enabled.

I have gone to Theme Settings>General setup>Page & Buttons and made sure CALLBACK IS SET TO CALLBACK PAGE.

I contacted Authorize.net and this is what they said:

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Ok, so there may need to be a relay url, I’m having a hard time finding any documentation for Premium press. You: | 10/9/2019, 5:38:41 PM
Do I have to get one from you? Omar | 10/9/2019, 5:40:21 PM
No, the relay url is a place the premiumpress would like us to direct the customers to after the transaction. Since it is a place that exists on the website, premium press would typically provide this Url. You: | 10/9/2019, 5:41:41 PM
Once I get from them, where do I put it on Authorize.net? Will that take care of the money being credited to the membership? Omar | 10/9/2019, 5:42:41 PM
Let me backtrack a bit here, I’ll explain what exactly needs to be done. It seems I was able to locate some information on their forums. Omar | 10/9/2019, 5:42:56 PM
What you are looking for is called a “Default receipt link url”. Omar | 10/9/2019, 5:43:28 PM
This is provided by Premiumpress,
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So how do I get this to work?
Do I need Default receipt link url?

Where is it?