How to remove the hash field in online cash registers?

Question

PHP

Hello, I need to apply payments on the site, so to speak, connect an online checkout.
I searched a lot of online cash registers and all of them have such a field in the api as sign for hashing the form.
I want to make it so that the user himself entered the amount manually in the field, and there was no specific amount.
the point is that I can't simultaneously learn the value from the text field in both html and php, so to speak, the hash does not match
the form. Is it possible to form such a form somehow? if there are people who will help I will be grateful, for a week now I can not find an answer to draw up such a form. All thanks for the help