I have used F-Secure for years and tonight I got this pop up on my screen, i have never encountered it before and would like to know if it is genuine.
Yes, this prompt / dialog is about to what it should be in recent builds. So, as such - genuine.
However, the reason and need for the restart may be more important / interesting to you. For example, which file / what detection / where located. A restart is probably only requested when really needed (so it shouldn't appear again after once. Otherwise it may mean the presence of tricky files).
You could try to request assistance with their official Support channels (for example, web-chat): Contact F-Secure support | F-Secure