question

You add the tip when you pay for the charge/order.

https://docs.clover.com/docs/using-the-clover-hosted-iframe (see Creating charge, this is when you add a tip)

https://docs.clover.com/docs/ecommerce-accepting-payments (see Adding tips to charges and orders)

I haven't worked with the iframe much, but you control the form markup, so you can add a tip input to the form. You can then submit the value of the tip input to your server where you create the charge (similarly to how you submit the token).