Summary
With either string concatenation or string interpolation (via template literals).
Here with JavaScript template literal:
function geoPreview() {
    var lat = document.getElementById("lat").value;
    var long = document.getElementById("long").value;
    window.location.href = `http://www.gorissen.info/Pierre/maps/googleMapLocation.php?lat=${lat}&lon=${long}&setLatLon=Set`;
}
Both parameters are unused and can be removed.
Remarks
String Concatenation
Join strings with the + operator:
window.location.href = "http://www.gorissen.info/Pierre/maps/googleMapLocation.php?lat=" + elemA + "&lon=" + elemB + "&setLatLon=Set";
String Interpolation
For more concise code, use JavaScript template literals to replace expressions with their string representations.
Template literals are enclosed by `` and placeholders surrounded with ${}:
window.location.href = `http://www.gorissen.info/Pierre/maps/googleMapLocation.php?lat=${elemA}&lon=${elemB}&setLatLon=Set`;
Template literals are available since ECMAScript 2015 (ES6).