Contact Form with drop-down subject (bonus: auto-selected based on GET/POST)
You can try hook_node_view() to pre-select default value of webform component on the base of referer.

function yourmodule_node_view($node, $view_mode){
    if($node-&gt;nid == [nid of webform]){
            global $base_path;
            $referer = $_SERVER["HTTP_REFERER"];
            $start = strpos($referer, $base_path) + strlen($base_path);
            $ref_local_path = substr($referer, $start);
            switch($ref_local_path){
                    case 'service_1_path':
                            $node-&gt;content['webform']['#form']['submitted']['&lt;component_name&gt;']['#default_value'] = 'option_1';
                    break;
                    case 'service_2_path':
                            $node-&gt;content['webform']['#form']['submitted']['&lt;component_name&gt;']['#default_value'] = 'option_2';
                    break;
            }
    }
...


If you are looking to do it via JS/JQuery that is another solution.