Calculated fields in forms

Hello, how do I show a calculated field inside a form?
In this case I have a list of Products and a form to Enter stock.
In the form, I need to show the value of the quantity multiplied by the cost in such a way that as the user enters the quantity, the Total Value field will show him the result of the multiplication, without having submitted the form.
I tried to do it with a Math column, but it does not allow me to show it, since it only allows me to access the fields of the Products table and not the Income table where I am entering the records.

Thanks for your help!!

You can’t perform calculations in a form because none of the data gets submitted to the table until you click on submit. I recommend creating a custom form that would allow you to do this.