Update plan-selection.mdx

Added ref to adding billing to URLs

Author: clairekinde11

src/content/docs/billing/billing-user-experience/plan-selection.mdx

@@ -119,3 +119,7 @@ You can configure Kinde to hide pricing tables in the sign up flow, but still sh
 1. Go to **Settings > Environment > Billing**.
 2. Switch off the **Show the pricing table when customers sign up** option. 
 3. Select **Save**. 
+
+## Add plan selector to registration URL
+
+Use your SDK or manually change URL params to incorporate billing. See [Add billing to URl or SDK](/billing/billing-user-experience/add-billing-to-url-sdk/)