From Xojo Documentation
Text of a message displayed as a help tag (tooltip).
The tip/tag is displayed when the user places the mouse on the control and leaves it there.
Separate multiple lines with EndOfLine to have multi-line help tags.
|Firefox does not support multi-line help tags.|
For better compatibility, you can try embedding a Line Feed into the help text like this:
Line 1 Line 2 Line 3
This code in the Shown event of a Button sets the help tag: