Changing the property box is only possibly by coding, there is no GUI switch or property to do this.
The best way is to draw the line with correct properties at all.
Click on the starting point or enter coordinates in command line.
Then enter @ANGLE,LENGTH in the command line.
E.g. @45,12.5 will create a line of length 12.5 at an angle of 45° from the start point.
The other option is to use geometry rules, e.g. draw a circle with center in the start point and a radius of desired line length.
Then you can trim the line with the circles circumference and delete the circle again.
Pardon me! Of course, you are right, it's my fault. My example was relative coordinates, not relative angle.
@45,200 drew a line which ended at X (startX + 45) and Y (startY + 200).
I'm glad that you figured this out yourself.