adds the ability to send a message to someone that is not your friend, through the "more" button on the steam profile.

SteamSendaMessage.user.js

// ==UserScript==
// @name         Steam Send a Message
// @namespace    MalikQayum
// @version      0.3
// @description  adds the ability to send a message to someone that is not your friend, through the "more" button on the steam profile.
// @author       MalikQayum
// @include      /^https?://steamcommunity\.com/(id|profiles)/*/
// @grant        none
// ==/UserScript==

if ( $J( "#btn_add_friend" ).length || $J( "#btn_accept_friend" ).length ) {
    var container = document.querySelector( '.popup_body.popup_menu.shadow_content' ),
        url = "javascript:OpenFriendChat( '"+g_rgProfileData.steamid+"', 0 );",
        image,
        element;

    if( container )
    {
        image = document.createElement( 'img' );
        image.className = '.popup_menu_item';
        image.src = 'https://steamcommunity-a.akamaihd.net/public/images/skin_1/icon_btn_comment.png';

        element = document.createElement( 'a' );
        element.href = url;
        element.className = 'popup_menu_item';
        element.appendChild( image );
        element.appendChild( document.createTextNode( '\u00a0 Send a Message' ) );

        container.insertBefore( element, null );
    }
}