prepopulate donation level with url

Resolved with two tips.

First:
I’ve put in footer.php

<script src="http://code.jquery.com/jquery-2.1.4.min.js"></script>
<script>
var dataObj = {};
var data = document.location.search.substr(1);

    $.each(data.split("&"), function () {
        var el = this.split("="),
                elName = decodeURIComponent(el[0]),
                elVal = el.length > 1 ? decodeURIComponent(el[1]) : null;
        if (!(elName in dataObj)) {
            dataObj[elName] = [];
        }
        dataObj[elName].push(elVal);
    });
    $(":input").each(function () {
        if (!($(this).attr("name") in dataObj)) {
            this.checked = false;
        }
    });
    $.each(dataObj, function (i, val) {
        $("[name='" + i + "']").val(val);
    });
</script>

http://www.site.com/form/?/?give-radio-donation-level=500.00 where “give-radio-donation-level” is the name of the input radio and “500.00” is the value of the radio.

And in the functions.php (or the plugin used for add functions)

with this: https://givewp.com/populate-donor-info-one-click/

I can use in url the amount.

In the end the complete url can be:

http://www.site.com/form/?give-radio-donation-level=500.00&amount=500.00

So the amount input is 500
The checked donation level is the corrispondent of 500 created in settings.