Heyhey,
I guess my question is very basic but I don’t get it, sorry …
How do I add a »LATIN CAPITAL LETTER SHARP S« to my font?
Thank you 
Add a glyph called “Germandbls” (note the capital G).
You can find that kind of info via Window > Glyph Info.
It worked, thanks!
I need more informations about this.
In an all-caps typeface, the German lowercase eszett (U+00DF) should by default look like a normally spaced double S.
So I need to add the current form ẞ, I could do that as stylistic alternate
Yes.