Inserting An Opening Note

I'm transposing a score that has a single eighth that opens the piece, which is then followed by the open repeat bar ( ||:) and then the score begins. It seems the only way I can create this structure is by inserting the note, followed by dragging the repeat bar line past it. The trouble is that MuseScore doesn't see the bar line as the beginning of a new measure, and thus won't allow me to write the first full measure in 4/4 time.

Here's the source:

How can I insert this note? MuseScore does not make it easy to go back and insert notes.