Is it possible to write grace notes before the bar line?
Select the grace note, open the properties panel (ctrl-8) and enable the property “before barline” 
Thank you! I don’t understand, that I didn’t found that!
You do not see it until you know it exists !!!
I have found that even when “before barline” is selected, the grace notes are appearing after the barline at the start of a new system. Is this intended behaviour?
Make sure you select the right-most grace note and activate the ‘Grace note before barline’ property. The grace notes will appear at the end of the previous system.
That did it, thanks! I was selecting a pair of grace notes and assumed it would set the property for both.