In the German-English dictionary that I have, it shows plural of nouns in suffix form. When it includes an umlaut also, it mentions it by having an umlaut over dash.
For an example from dictionary,
der Bogen (-̈) = curve or bend.
Here, -̈ shows that plural of Bogen would be Bögen.
But in my experience, umlaut doesn't cover the dash properly like in dictionaries. What should be the two proper combination of unicode characters for this? I have tried the hyphen as well as em- and en-dash with combining diaeresis (0x0308).