Hi,

Add a field to your XPage and a button. Mark the field as required. By default if you click a button, all fields will be validated first, so on the field's onclick event, you can directly add the code to run the agent ( session.getAgent("nameOfAgent").run() ) and save the document ( doc.save() ). 

If you need to pass parameters to the agent, use the .runWithDocumentContext() method.

I made some assumptions in my answer, so you might to tweak it for your situation.

Mark

if you want to use partial refresh, you can code the client side javascript of the button to do the field validation and return a true or false.  The Server Side button code will only execute if the client side code returns true.  So you could call the agent from ssjs code upon the successful completion of the csjs code