It isn't as intuitive as you asked for (or I would like) but here the process I found today (Feb 21, 2024 -- Studio One Pro v6.5.1)
With the Song open:
From the Song dropdown select "Song Setup" [default keyboard shortcut <ctl+.>
In the Song Setup window, select the "General" tab.
This should show boxes for "Song Start" and "Song End" that are editable. Simply type in whatever number you want, click Apply, and OK.