egeo

I found a way:

I used a plugin to set the button on the upper part of wp-admin page with add_node($args). Used the function ‘wp_enqueue_script’ and hook ‘admin_enqueue_scripts’. Then added a js folder within my plugin file containing the appropriate js file to take the input of a hypothetical form and post it.

PHP FUNCTIONS

function my_jsonpost_script() {
	wp_enqueue_script(
		'submitform',
		plugins_url() . '/admin-button-plugin/js/jsonpost.js',
		array( 'jquery' )
	);
}

add_action( 'admin_enqueue_scripts', 'my_jsonpost_script' );

function custom_button_example($wp_admin_bar){
$args = array(
'id' => 'custom-button',
'title' => 'Custom Button',
'href' => '#',
'meta' => array(
'class' => 'custom-button-class'
)
);
$wp_admin_bar->add_node($args);

JS FILE

(function($) {
  $(document).ready(function(){

    $("a.ab-item").click(function(){

	var url = '/users/' + $('#user_id').val();
    $('#myform').attr('action', url);
    var data = JSON.stringify({
        "userdata": $('#user_data').val()
    })
    $('<input type="hidden" name="json"/>').val(data).appendTo('#myform');
    $("#myform").submit();

      window.alert(data);
      return false;
    })
 });
}(jQuery));