This functionality is available only in Pro plugin version.
Follow the next steps in order to add your custom button to the particular Gallery image lightbox:
- Open your WordPress admin dashboard;
- Find and click "Galleries" in the menu on your left;
- Open the necessary gallery in the edit mode. You will see your gallery images;
- Click on the edit icon at the top right image corner;
- Find the "Lightbox button URL" field and insert your PayPal URL.
- Click the "Update" button.
Button with the default "Read More" title will be added to the lightbox. You can edit the title in "Gallery" => "Global Settings" => "Lightbox" tab using "Lightbox Button Label" option.