For example:

Set MapFrame = Plot.Shapes.AddPostMap(SurferApp.Path+"\samples\demogrid.dat", LabCol:=3)
Dim PostLayer As Object
Set PostLayer = MapFrame.Overlays(1)

'Sets the label format properties. For all options, see LabelFormat object.
PostLayer.LabelFormat.NumDigits = 1
PostLayer.LabelFormat.Type = srfLabFixed

'Sets the label font properties. For all optiosn, see FontFormat object.
PostLayer.LabelFont.Face = "Comic Sans MS"
PostLayer.LabelFont.Size = 14
PostLayer.LabelFont.ForeColorRGBA.Color = srfColorBlue
PostLayer.LabelFont.Bold = True

'Sets the angle for the label text
PostLayer.LabelAngle = 30

'Sets the label position relative to the symbol
PostLayer.LabelPos = srfPostPosUser
PostLayer.LabelPos = srfPostPosUser

'Sets the X and Y offset for custom label positions relative to the symbol. This is only active if the LabelPos above is set to User (User Defined).
PostLayer.LabelXOffset = 0.02
PostLayer.LabelYOffset = 0.01

'Sets the 3D label line properties. See LineFormat object for all line property options.
PostLayer.LabelLine.ForeColorRGBA.Color = srfColorGreen

'Sets the length of the 3D label lines
PostLayer.LabelLineLength = 0.5

'Sets the plane for the labels
PostLayer.LabelPlane = srfPostScreen

See also the attached script PostMap.bas, which can also be found in the \Samples\Scripts\ folder in the Surfer installation directory.

Updated November 1, 2018