How to make a form on which the customer can type in the amount of the payment?

We are a storage facility. We need to collect credit card information from our customers for their monthly rent amount, which can change and fluctuate.  I tried to make a credit card form and it wanted me to have set prices or products.  Can you show me how to make a secure credit card form on which the customer can type in the amount of the payment?

On your selected payment field integration settings, instead of selecting "Sell Products" as payment type, you will select "User Defined amount".

Here's a screenshot:

Feel free to let us know if you need any further assistance.