Entering umlauts via special key sequences (Wikipedia):
On Microsoft Windows keyboard layouts that do not have umlaut characters, these characters can be entered by pressing the left Alt key, and entering the full ANSI numeric code on the numeric keypad, provided that the compatible code page is used as a system code page. You can also use numbers from Code page 850; these lack a leading 0.
Character-------ANSI Code--------------CP850 Code
ä---------------Alt+0228---------------Alt+132
ö---------------Alt+0246---------------Alt+148
ü---------------Alt+0252---------------Alt+129
Ä---------------Alt+0196---------------Alt+142
Ö---------------Alt+0214---------------Alt+153
Ü---------------Alt+0220---------------Alt+154
ß--------------------------------------Alt+225
On a computer running MacOS umlauts can be entered be pressing option-u, followed by the vowel to have an umlaut above it. X-based systems with the Compose key can usually enter umlauted characters by typing Compose, " followed by the letter.
