Mark Text On Inactive Form With 1st Klick

imagine following situation 2 forms, one form with some buttons and another form with a RichTextBox on it, if i click on the buttons the text in the RTB changes. Now i want to mark some words in the RTB with the mouse. The form with the buttons is the active form, if i click in the RTB and try to mark something, the form gets focus and comes to the front but if i move the mouse nothing happens. I have to perform a second click at first to mark something.I already tried a lot of things like setting focus to the form if the MouseButtonDown event fires, but all of that doesn't work. The only possibility that works if i set focus to the form when ever the mouse moves over the form, but this is annoying, cause everytime i move the mouse on the form the form pops to the front.How can i manage to mark the text with one simple click? Applications like OpenOffice or Notepad++ are able to do this as well.