This field only applies if you have enabled Adding new products to the subscription from the Product and Shipping Management section found in Customer Portal and then Configuration. If so, this field will allow you to change the Add Product text (highlighted below).
To enable this feature, carefully follow the steps outlined in the full article above.
Step 1: Open the Customer Portal Configuration
From your Appstle Subscriptions dashboard, go to the left menu and click on Customer Portal.
At the top of the Customer Portal page, select the Configuration tab.
Refer to Image 1: This shows where to find and select the Configuration tab under Customer Portal.
Step 2: Enable 'Add Products to Subscription'
Scroll down to the Product and Shipping Management section.
Check the box labeled Let customers add more products to their existing subscriptions.
Refer to Image 2: This highlights the specific checkbox you need to enable under Product and Shipping Management.
Step 3: Save Your Changes
After enabling the option, click Update at the top right corner to save your settings.
Once you've enabled this setting, the “Add Product” button will appear for your customers, and you can customize its label via the Add Product Button Text field.