By the simple trick of using "Record Macro", I was able to generate the following code (in Excel 2013):
Selection.ShapeRange(1).TextFrame2.TextRange.Characters.Text = "This is my shape" With Selection.ShapeRange(1).TextFrame2.TextRange.Characters(1, 12). _ ParagraphFormat .FirstLineIndent = 0 .Alignment = msoAlignLeft End With With Selection.ShapeRange(1).TextFrame2.TextRange.Characters(1, 12).Font .NameComplexScript = "+mn-cs" .NameFarEast = "+mn-ea" .Fill.Visible = msoTrue .Fill.ForeColor.ObjectThemeColor = msoThemeColorLight1 .Fill.ForeColor.TintAndShade = 0 .Fill.ForeColor.Brightness = 0 .Fill.Transparency = 0 .Fill.Solid .Size = 11 .Name = "+mn-lt" End With
The first line of the above seems to be enough to enter the text into the shape; the rest, apparently, merely explicitly set the default attributes.