Make a payment link
Put in the amount, copy the link, send it to the client. The link opens a page with a Pay button that fills the amount into their UPI app.
The link
Open the link yourself once before you send it, to check the amount reads the way you expect. Do not try to pay it yourself — UPI refuses a payment where the payer and the payee are the same person, and the apps report that as a vague decline. To test it for real, ask someone else to pay ₹1 from their own phone.