At the beginning of your piece, using the meter pop over, type “6/8,1.”
Without the quotes.
And if you want to delete a pick up bar that you’ve previously created, you would click on the first full bar, add the desired meter, go back to the pickup bar, shift-B, and type -1.
Will this add the 5-eighth-note bar at the end of the flow?
No. To do that you’d need to select the penultimate bar line, type Shift+M and then type 5/8 into the popover. You can then hide that time signature from the bottom panel.
Another option is to move the caret to the last eighth note position of the last bar and simply enter a final barline (shift+B, then type |] ).