json - How to do a GET request call in Sencha Touch?

Question

My code is this: I want to pass user name and password to a get request API which will return JSON Output. I am accessing the web services which is hosted in some other domain.

Ext.data.JsonP.request({
    url: serviceUrl,
    callbackKey: 'callback',
    params: {
        username: username,
        password: password
    },
    success: function(response, opts){
        var output = result.responseText;
        var parsedJSON = Ext.JSON.decode(output)

        if((parsedJSON.errorMessage)== null){
            Ext.Msg.alert('Authenticated');

        }
        else{
            Ext.Msg.alert('Not Authenticated');
        }

    },
    failure: function (result, request) {
        Ext.Msg.alert('Failure'+result.responseText);
    }
});