If you want to use pencil, brush, smear finger or other brush tools:
Hold shift, press and hold mouse button, release shift (or keep holding it if you want 45°), move to the end point, release mouse button.
Or you can use "create line". That allows to correct positions, colors, use an arrow as a line style, add shadow effects...
Create line.png
And for very special needs, there's "layout > image > path painting" to combine vector and brush.
You do not have the required permissions to view the files attached to this post.
----------------
Herr Doktor, ich bin mir ganz sicher, ich habe Atom! /Doctor, doctor, I'm sure, I've got atoms!