Hi Richard,
Thanks for posting in the Community!
There isn't a discount or percentage field in the EZ Map Field for custom forms in QuickBooks Online. However, you can add custom fields if you are using the Essentials and Plus version. And map them as <custom1> for Custom Field 1.
You can check these links below to learn more about adding custom fields and customising sales forms:
Post again in the Community if you have further concerns with customising your sales forms.