How can I have a payment form that would allow users to enter any amount?

How can I have a payment option that allows a user to enter any amount they choose to enter as a form of payment?

You have to use one of our payment tools, Paypal, for example. In the configuration, select "Collect Donations/Custom Amount Payments" from the list:

Please make sure to uncheck the donation option to be able to collect a regular payment.