how to assign extended ASCI keys to empty keyboard characters in LO Writer using Windows 8

In XP it was possible to assign UMLAUT to empty keys. In Windows 8 this feature seems no longer to exist. Example: Ctrl+Alt+a = UMLAUT ae

Do you mean a feature of LibreOffice or a feature of the operating system? How was it done in XP?

What keyboard layout do you use? US-International has similar shortcuts for Umlaut and acented characters.