you can use javascript as the scheme of your submit button's url to run javascript

i personally would just add a button element to your portlet's form and add your code there

make sure to use the type attribute if you create your own button, its literally inside a form element and by default behaves like a submit button

how can i get the submit button of my portlet for call that function

portlet.setSubmitButton({
      url: "output",
      label: "Submit",
      target: "_hidden",
    });               function _print_api() {
    console.log("testing");
    // print_shipping_label(document.forms["main_form"].orderid.value, id);
  }

print_shipping_label doesnt exist in a portlet

that said, use Portlet.addField to add an linline html field so you can add your own button

like this ?

use Field.defaultValue to set the value of the html

It is clientside

That said, I told you in the beginning, I expect that you will need to change your approach

print_shipping_label is an internal netsuite function that is only available on certain pages

If you can, you would want to make a suitelet that creates you shipping labels and use that servlet with your button

If you want to use N/render, then you will be creating a new file object

You can load an existing file if it already exists