Abandonment Checkout Flow redirect to link failing

Hi, I have started to set up my abandonment checkout series Flow.  I created my first email as TXT email and wanted to use a link for the customer to use that will take them straight back to their abandent check out. Once i press preview email the link doesn’t get me anywhere! I have used the following links:

{{ event.extra.checkout_url|default:'' }}

{{ event.extra.responsive_checkout_url|default:'' }}

{{ event.extra.checkout_url }}

{{ event.extra.responsive_checkout_url }}

Why am i not successful?

Would appreciate some assistance. Also options when pressing the link button to insert link are very limited i cant choose https or email or others its only saying URL.

I have a Shopify store!

Thank you