How to set the value for a custom field in a list?

I am editing the script and HTML for a Content Editor web part. My code opens up a Modal Dialog when a user clicks a link on my page. In the dialog, the custom fields I've defined for the list appear.

I use the follow code to get the current logged in user's name:

function getWebUserData() 
{  
    context = new SP.ClientContext.get_current();  
    web = context.get_web();  
    currentUser = web.get_currentUser();  
    currentUser.retrieve();  
    context.load(web);  
    context.executeQueryAsync(Function.createDelegate(this, this.onSuccessMethod), Function.createDelegate(this, this.onFailureMethod));  
    var userObject = web.get_currentUser(); 
    // this is how to get the user name --> userObject.get_title();
}

This code works fine and I only post it here as part of my question.

So now that I can get the current logged in user's name, I need to add it to a custom field in the dialog window. In other words, when the dialog appears, and if there are three fields presented for the user (such as "Your Name", "Priority" and "Message") I need to auto populate the "Your Name" field in the dialog so the user does not need to fill it out.

So, I can get the user data, but how do I set the field in the dialog window with it? I would like to do this in Javascript.

Please help. Thanks

----UPDATE----

Here is part of my code, which might help resolve the issue:

<script type="text/javascript" src="/Extra%20Documents/jquery-1.11.2.js"></script>
<script type="text/javascript" src="/Extra%20Documents/spjs-utility.js"></script>
<script type="text/javascript"> 

ExecuteOrDelayUntilScriptLoaded(getWebUserData, "sp.js");  
var context = null;  
var web = null;  
var currentUser = null; 

function getWebUserData() 
{  
    context = new SP.ClientContext.get_current();  
    web = context.get_web();  
    currentUser = web.get_currentUser();  
    currentUser.retrieve();  
    context.load(web);  
    context.executeQueryAsync(Function.createDelegate(this, this.onSuccessMethod),     Function.createDelegate(this, this.onFailureMethod));  
}

function ContactUsOpenDialog(strPageURL)
{
    var userObject = web.get_currentUser(); 
    var options = 
    {
        title: "Contact Us",
        width: 750,
        height: 550,
        url: strPageURL
    };

    //
    // I suspect that there is a problem in the code below
    //
    fields = init_fields_v2();
    setFieldValue('Your_x0020_Name','My custom value');
    SP.UI.ModalDialog.showModalDialog(options);
}
</script>

// After several lines of HTML

<a href="#" onclick="javascript:ContactUsOpenDialog(&#39;http://ift.tt/1IJUpmW Us/NewForm.aspx?RootFolder=&#39;);">

Can anyone see the issue?