Let customer choose it's own amount on product page

-- HikaShop version -- : 5.1.2

Hi Hika team,

I sell gift cards on my website.
You can choose from an amount of €25.00, €50.00, €75.00 and €100.00, but I also want to give customers the opportunity to choose an amount themselves.
I have created an item field for this, but I would like the custom amount to be calculated in the cart and to be paid for directly.
After payment I will make a custom gift card and send it by email.

Is there a way to implement this?

Edit: Also I noticed when I choose the item custom field the add to cart button disappears.

Thanks in advance.

Thank you Nicolas.

Is it also the intention that I place the product (Kadobon) itself in the Donations category?