So far I know because I am just a user (and a fan) of Safesky, you go to the preferences, then you click above on your name and than you change your nickname.
I wonder if it is not at the “call sign” level in My Aircraft that we must act.
In any case for me, what appears on the screens is the call sign that I put in My Aircraft.
I have the answer. To be seen on glidernet, you have to define an aircraft with a call sign AND the Public Profile must be on.
You may define a call sign you want, for ex. FVA123
Effectively. To see your “call sign” on the glidernet (OGN), your profile must be set in “public”. Otherwhise, you just see a trafic but without the call sign.