How to get an input value without refreshing with twig? One way to read a dynamically inserted value is to refresh the page and get it in a twig variable through the url, something like this:
{% set amount = craft.app.request.getQueryParam('amount') %}

But I am looking for a solution to get a value inserted into an input without refreshing the page, of course, after sending the form (i thing). Do you know if thats possible? Maybe with javascript.
Yes you would do this using Javascript so is a little outside the scope of this StackExchange site but here's something that might help get you moving...
Getting the value is easy with something like
document.querySelector('#myElement').value

You'd need to trigger that in some way, perhaps when the user leaves the field. So (assuming your input has an ID of &quot;myElement&quot;...
document.querySelector('#myElement').addEventListener('blur', function(el){
    let theValue = el.value
})

What this is doing...
Get the element on the page with an ID of 'myElement'
Adds a listener to it for the 'blur' event (when the user leaves the element)
When that happens it executes a function and passes the element into it
Sets a variable (theValue) with the value of the element.
You now have a javascript variable named 'theValue', but what do you want to do with it?