German characters on US-International keyboard layout
AltGr+s. You could get help in Windows Keyboard Layouts, in particular at United States-International (with AltGr pressed):
How do I enter the ß (sharp s)?
ß Right Alts Esszett (sz ligature)
Source Help with Microsoft Keyboards US-International | School of Languages, Cultures, and Race | Washington State University
Personally i use this layout https://eurkey.steffen.bruentjen.eu/
It's basically the US layout with all of the umlauts (and anything else) you might need.
äöüß would be AltGr + a/o/u/s.