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I am not sure if I have understood your question correctly. You should be able to add an umlaut directly in the form title. Do you mean to say that you are unable to edit the form title?
I did test this and I was able to add the umlaut in the form title. Please be noted that if you are using your form's direct URL, you will have to add the title in the "Page Title" of your form's preferences. Please check the screenshot below:
Hope this helps.
Do get back to us if you mean to ask something else.