ContainerControl.Tooltip
From Xojo Documentation
Property (As String )
aContainerControl.Tooltip = newStringValue
or
StringValue = aContainerControl.Tooltip
New in 2019r2
Supported for all project types and targets.
or
StringValue = aContainerControl.Tooltip
New in 2019r2
Supported for all project types and targets.
Text of help message displayed as a Windows or Linux "tip" or macOS help tag.
The tip/tag is displayed when the user places the mouse on the control and leaves it there.
Example
Me.Tooltip = "This is a tooltip."