fix: set default value on dropdown with only one option

Author: justynoh

packages/backend/src/apps/paysg/actions/create-payment-form-submission-subscription/index.ts

@@ -89,6 +89,7 @@ const action: IRawAction = {
         },
       ],
       showOptionValue: false,
+      value: 'monthly',
       required: true,
     },
     {