Custom Button to Start Checkout with Subscription Product

I currently have a replenishment reminder flow that lets people who ordered a one-time product know that they are going to run out 7 days before they’re on empty. I have a custom button in that flow email that automatically adds whatever one time product they’re running out of to checkout. Here is the button that adds the one time product to checkout: {{ organization.url|trim_slash }}/cart/{{ event.extra.line_items.0.variant_id|default:'' }}:{{ event.extra.line_items.0.current_quantity|default:'' }}

I want a second button that says subscribe now below ‘never run out!’ and I would like that button to add the subscription product (only) to checkout. Can someone please help me with what the link for that button would be? I am on Shopify checkout and I use recharge for subscriptions.