Actually there is a catch, you need at least one entry point in the client script, even if is blank... Suitelet Code

form.addButton({                              
  id: "custpage_button",                                   
  label: "Click Me",                                            
  functionName: "onButtonClick"                                  
});
form.clientScriptModulePath = 'SuiteScripts/clientScript.js';

Client Code

/**
*@NApiVersion 2.0
*@NScriptType ClientScript
*/
define([], function() {    
    function onButtonClick() { alert("You clicked a button."); }
    function pageInit() {}; // needed by NetSuite
    return {
        onButtonClick: onButtonClick,
        pageInit: pageInit
    };
});