Quartina

After the first three eighth notes are in, fill the rest of the measure with a dotted half note or rest, click on it, and press Ctrl+4.

No need to first enter a dummy note and then select it. While still in note input mode, after entering the first three notes, simply select dotted half as the duration then press Ctrl+4. However, note this will enter the tuplet using the more standard notation of four *quarters*, not four *eighths*. The notation shown is not recommended by modern professional editors, so I would encourage you to simply use the quarters as MuseScore defaults, unless your music is being read only by musicians living in the same centiry that edition was originally published (when that notation was more common) :-). If you do for some reaosnm want to copy that older style of notation, you'd need to enter the tuplet using Notes / Tuplets / Other, and choose 4 / 6 as the ratio (4 eighths in the space of 6).