Add Start Notes - whilst maintaining original eight bars in line one

Select these nine bars and apply one or several times the function 'decrease layout stretch', simply by pressing this bracket: '{'.
Musescore adapts the bar length to the content of the bars. That is o.k. because it should remain readable.
E.g. if you have bars with a lot of semiquavers, you will not be able to have many bars in a line.

Methods to get more bars in a line are:
- Reduce the staff spacing (sp) a little in the format/page settings.
- choose a different page size, such as A3. When exporting to a PDF and printing, your PDF reader may reduce the page to A4.

I used the layout stretch function for the attached file.

Click on the very first note of your score then, while holding the Shift key, click on the last note of the first measure on the second line (the last note in the measure labelled #8). Press { a couple of times to decrease the layout stretch of those measures until they fit onto the same line. (Alternatively, use menu item: Format > Stretch > Decrease Layout Stretch.) This will compress the pickup measure and the following 8 bars onto line one. See:
https://musescore.org/en/handbook/3/layout-and-formatting#layout-stretch

If desired, you can take it a step further and go to menu item: Format > Style > Score and untick the box 'Enable Indentation on first system' to give the notes on that first line more room - and looking comparable to the rest of the piece.