Step 1:

Open your popup.
Click the email form.

Step 2:

In the left menu, click "Add new field"

Step 3:

In the field type list, select "Hidden field".

Step 4:

In the "Name" field, input "URL".

Save your popup.

Step 5:

Paste the following script between the end of your WisePops tracking code and the closing </script> tag

(function () {
    function fillUrlInHiddenField() {
        var hiddenField = document.querySelector('.wisepops-content input[name="URL"]');
        if (hiddenField) {
            hiddenField.value = window.location.href;
        }
    }

    function handleMouseOverPopup(e) {
        if (e.target.closest('#wisepop-XXXXX .wisepops-content')) {
            fillUrlInHiddenField();

            // Execute only once
            document.removeEventListener('mouseover', handleMouseOverPopup);
        }
    }

    document.addEventListener('mouseover', handleMouseOverPopup);

    // IE 11 Compatibility
    // https://developer.mozilla.org/en-US/docs/Web/API/Element/closest#Polyfill
    if (!Element.prototype.matches) {
        Element.prototype.matches = Element.prototype.msMatchesSelector ||
            Element.prototype.webkitMatchesSelector;
    }
    if (!Element.prototype.closest) {
        Element.prototype.closest = function(s) {
            var el = this;

            do {
                if (el.matches(s)) return el;
                el = el.parentElement || el.parentNode;
            } while (el !== null && el.nodeType === 1);
            return null;
        };
    }
})();

You should end up with something close to this:

Step 6:

In the URL bar, copy the ID of the popup

In the script you just pasted, replace XXX by the ID of your popup

#wisepop-XXXXX

In my example:

#wisepop-139719


You're done!

Note: 

If you want this script to work for any popup, you can remove the #wisepop-XXXXX part in the code:

(function () {
    function fillUrlInHiddenField() {
        var hiddenField = document.querySelector('.wisepops-content input[name="URL"]');
        if (hiddenField) {
            hiddenField.value = window.location.href;
        }
    }

    function handleMouseOverPopup(e) {
        if (e.target.closest('.wisepops-content')) {
            fillUrlInHiddenField();

            // Execute only once
            document.removeEventListener('mouseover', handleMouseOverPopup);
        }
    }

    document.addEventListener('mouseover', handleMouseOverPopup);

    // IE 11 Compatibility
    // https://developer.mozilla.org/en-US/docs/Web/API/Element/closest#Polyfill
    if (!Element.prototype.matches) {
        Element.prototype.matches = Element.prototype.msMatchesSelector ||
            Element.prototype.webkitMatchesSelector;
    }
    if (!Element.prototype.closest) {
        Element.prototype.closest = function(s) {
            var el = this;

            do {
                if (el.matches(s)) return el;
                el = el.parentElement || el.parentNode;
            } while (el !== null && el.nodeType === 1);
            return null;
        };
    }
})();
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