You must register a domain for one of your customer, but you are the one who sets the billing side.
To do this, you must first create an additional contact via your customer account: https://kb.planethoster.com/en/guide/contactssubaccounts.
Then you can order the domain via the page: https://my.planethoster.com/domains/new.
In the form, you can change “Domain Owner Information” to that of your customer.
Place the order and the domain owner will be in the name of the additional contact.