I have read this post: How do I insert an umlaut on the u in ubuntu? and i managed to get the Umlaute:
Activate the Compose Key like described above in Tweaks. (i have chosen Right Alt) and then enter in the text editor:
AltGr + a
and then "
for ä
But how do i get it work in the Web text input like in Stackoverflow? Pressing the combination in the Webinterface just prints out a