Custom Button modal window.open

Question

I have a custom button and I am using a modal to prompt the user for input. There is a button that opens a new window url. The variable for the url is not formatting correctly.

custom button javascript:

{!REQUIRESCRIPT("/soap/ajax/34.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/34.0/apex.js")}

function insertScript(a) {
    alert(a);
    var body = '<h1>Select Office Name</h1>: ';
    var list = sforce.apex.execute("DQCustomButtonUtils", "getProviderFacilities", {
        id: "{!Contact.Id}"
    });
    var offices = JSON.parse(list);
    var box = new SimpleDialog("hersh" + Math.random(), true);
    parent.box = box;
    box.setTitle("Please Select Service Office");
    box.createDialog();
    box.setWidth(350);
    body += '<select>';
    for (var office in offices) {
        body += '<option value="' + offices[office].ww_Entity_Guid__c + '">' + offices[office].Name + '</option>' + '<br>';
    }
    body += '</select><br>';
    body += "<button class='btn' onclick='window.parent.box.hide(); return false;'>Cancel</button>";
    body += "&nbsp;";
    body += '<button class="btn" onclick="window.open("' + a + '"); return false;">OK</button>';
    box.setContentInnerHTML(body);
    box.setupDefaultButtons();
    box.show();
}
var a = sforce.apex.execute('DQCustomButtonUtils', 'getProviderWindwardUrl', {
    id: '{!Contact.Id}'
});
insertScript(a);

Alert showing variable a's url. This is correctly formatted:

How the button is actually written to the screen

<button class="btn" onclick="window.open(" http:="" localhost:53700="" provider.aspx?id="053659cc-cdde-45d6-826e-95cb787c005e&amp;effdte=2003%2f10%2f01&amp;termdte=12%2f31%2f9999&quot;);" return="" false;"="">OK</button>

Answer 1

I think the slashes ("/") in the "a" variable must be escaped. If you look at the way the button is being rendered, you'll see that where a slash is found the string "breaks up".

Answer 2

I have written a simple solution for rich modals on standard page layouts which would simplify your code here http://www.codebycody.com/2016/11/custom-html-dialog-box-on-standard-salesforce-page-layouts.html