Showing cross-staff melody with dotted lines

I've usually seen a solid line. If this is acceptable to you then use a glissando, uncheck show text and uncheck play in the inspector. You can then double click it to put it in edit mode and use shift+arrows to move the end to the correct staff and note. The line will automatically update.

If you insist on a dotted line then use the plain line out of the lines palette, put it in edit mode, drag the ends where you want them and mark the line style as dotted in the inspector. You need to do this after the rest of the score is complete because other edits may affect where the line moves.