From Xojo Documentation

Property (As Double )
aiOSLayoutConstraint.Offset = newDoubleValue
DoubleValue = aiOSLayoutConstraint.Offset

Supported on Mobile (iOS).

The offset that can be applied to the rule.

Sample Code

Change the offset (in this case the width) or an existing named constraint:

// "TAWidth" is a width constraint for a TextField that has been given
// a name in the auto-layout Inspector properties.
Var c As iOSLayoutConstraint = Self.Constraint("TAWidth")
c.Offset = 200