[Zubersoft]

If you select an annotation with the selection tool, you will see a bar pop up with icons. There is an icon on there for the nudge window - tap that, and if you want the nudge window to stay up, tap the pin icon.

Mike

[Skip]

You can get it back by making a line [or other annotation] > 'select' icon, the large solid arrow head >  select the line which will bring up a box with 5 icons > select the tilted square, 2nd from the left, will bring up the nudge tool. Make in stick by selecting the 'pin' icon.
If you have annotations already, tap one then tap the tilted square and pin.