You question is not clear, further explanation would help. For example, does your application make use of different windows for each chat ? Does each window have it's own "Send" button. Try and explain more concisely what your application does first before stating the problem.
however if you want a window to be come control to be automatically selected when a window is focused. i think you should listen to the Window Activated event of the window. You shouldn't need a timer for that.
public Form1()
{
this.Activated += new EventHandler(Form1_Activated);
}
void Form1_Activated(object sender, EventArgs e)
{
}