How to simulate a 'shift click'

Hello out there,
I can simulate a simple click but fail at a 'shift click'.
Below is what I tried:
---
package require Tk
foreach ch [winfo children "."] {destroy $ch}

# create a test app
set btn [ttk::button .btn -text "Press me"]
pack $btn
# add some bindings
bind $btn <Button-1> {puts "Simple click"}
bind $btn <Shift-Button-1> {puts "Shift click"}

focus $btn
set x [winfo x $btn] ; set y [winfo y $btn]
# place the mouse
event generate "." <Motion>  -x [incr x 50] -y [incr y 10] -warp 1

event generate $btn <ButtonPress-1>
after 100 ; event generate $btn <ButtonRelease-1>

# now a 'shift click'
if 1 {
event generate $btn <KeyPress> -keysym Shift_L
} else {
event generate $btn <Key-Shift_L>  
}
event generate $btn <ButtonPress-1>
after 100 ; event generate $btn <ButtonRelease-1>
event generate $btn <KeyRelease> -keysym Shift_L
---
All I ever get is twice 'Simple click' - no matter whether I use
<Key-Shift_L>  or the longer form, wether I generate the keypress for
$btn or  ".". I am out of ideas what to try next.
Any help will be greatly appreciated
Helmut.
PS: This is on Windows with Tcl 8.6.10