Hard to say since I've done have a scanner and have only done this a few times. However, most scanners simulate key presses, and key presses should essentially queue up for the application to consume them. Just trying this manually I can see that several of the first keys I press before the dialog has opened are lost. For example, I just press Ctrl+A to open the "Add Players" dialog and quickly typed my own name, and only the final "y" showed up in the "Search for" field.
I believe most scanners recognize certain characters to have special meaning, such as a pause. I would recommend adding a pause between the Ctrl+A (or TAB if that's what you're using) and the text to be searched. Probably a half second to a second would be all that's needed.