From Xojo Documentation

Property (As Double )
aclass.BorderOpacity = newDoubleValue
DoubleValue = aclass.BorderOpacity

New in 2019r2

Supported for all project types and targets.

Indicates the level of opacity.

Border As Double

Degrees of transparency is currently supported only on macOS and Windows. On other platforms, the border is either visible (100%) or invisible.

Sample Code

The following code adds a border to an ArcShape:

Var a As New ArcShape
a.ArcAngle = 1.57
a.StartAngle = -1.57
a.BorderOpacity = 100
a.FillColor = RGB(255, 0, 127)
g.DrawObject(a, 100, 100)