ServiceNow provides a client-side API called GlideModal but the documentation is focused around displaying records or lists from ServiceNow. There are times where you'd like to use a custom form with just a few options. In this example, we're adding a UI Action button to the catalog task form so when a task is cancelled we can either:

• Closed skipped - effectively cancelling just this task
• Closed incomplete - close this and all other tasks along with the requested item itself

In this way, we're giving our users the ability to clearly determine what they're cancelling. Just one particular task or the whole thing.

UI Actions allow you to run code client-side and server-side, which is very handy.

The key to getting some flexibility with your modal dialog is the renderWithContent which allows us to craft our own HTML and the window.changeTaskAction which allows us to interpret whatever happened on our modal as the user interacted with our options.

////////////////////////////////////////////////////////////////
//Client Side: Dialog box with choices
////////////////////////////////////////////////////////////////
function cancelDialog(){

    var gm = new GlideModal('cancelTask');
    //Sets the dialog title
    gm.setTitle('Cancel Task'); 
    //Set up valid custom HTML to be displayed
    gm.renderWithContent('<div style="padding:15px"><p>What action do you want to take?</p><p><select name="cancellation" id="taskCancellation" class="form-control"><option value="cancelOnly" role="option">Cancel this task but keep the requested item open</option><option value="cancelAll" role="option">Cancel this and all other tasks, closing the requested item</option></select></p><div style="padding:5px;float:right"><button style="padding:5px;margin-right:10px" onclick="window.changeTaskAction(this.innerHTML,jQuery(\'#taskCancellation\').val())" class="btn btn-default">Abort</button><button  style="padding:5px" class="btn btn-primary" onclick="window.changeTaskAction(this.innerHTML,jQuery(\'#taskCancellation\').val())">Cancel Task</button></div></div>'); 

    //We'll use the windows object to ensure our code is accessible from the modal dialog
    window.changeTaskAction = function(thisButton, thisAction){

        //Close the glide modal dialog window
        gm.destroy();

        //Submit to the back-end
        if(thisButton=='Cancel Task'){
            if(thisAction=="cancelAll"){
                g_form.setValue('state',4);//Closed Incomplete -- will close the Requested Item and all other open tasks
            }else{
                g_form.setValue('state',7);//Closed Skipped -- will only close this task
            }
            //Regular ServiceNow form submission
            gsftSubmit(null, g_form.getFormElement(), 'cancel_sc_task');
        }
    };
    return false;//prevents the form from submitting when the dialog first loads
}

////////////////////////////////////////////////////////////////
//Server Side: Dialog box with choices
////////////////////////////////////////////////////////////////
if (typeof window == 'undefined')
    updateTask();

function updateTask(){
    //Runs on the server
    if(current.state==7){
        //closed skipped so simply update this one record
        current.update();
    }else{
        //closed incomplete so update all associated records to close the requested item entirely
        current.update();

        //And now we'll cancel any other open tasks along with the requested item
        if(!gs.nil(current.parent)){
            //Close siblings
            var otherTasks = new GlideRecord('sc_task');
            otherTasks.addEncodedQuery('request_item='+current.request_item+'^stateIN-5,1,2');
            otherTasks.query();
            while(otherTasks.next()){
                otherTasks.state = '4';
                otherTasks.update();
            }
            //Close parent
            var ritm = new GlideRecord('sc_req_item');
            if(ritm.get(current.parent)){
                ritm.state = '4';
                ritm.stage = 'Cancelled';
                ritm.update();
            }
        }
    }
}

Code like this can be easily retrofitted to provide you with a good template for interacting with users before you save your record.

If for some reason the attached UI Action doesn't work, toggle the "isolate script" field (save it on then off) and it should run just fine.

Have fun.