Julie here from Shopify Support.
You can get your preview link through your Shopify admin, by heading to Online store > Themes. Next to the theme you're wanting to share, select Actions > Preview. From there, you'll notice a preview bar at the bottom of the page. Click Share preview and Copy link to copy the preview link to your clipboard. You can then share this link with your payment processor, who will be able to view your online store before it's launched.
Since you haven't yet launched your store, I wanted to pass along our general checklist. This will help you ensure you've completed all the steps necessary prior to launching.
I hope this helps! Let me know if you run into any issues sharing your theme preview link.