If you know the name of the Text Box, you can reference it this way:
ActiveSheet.Shapes.Range(Array("TextBox 1")).Select
If you don't know the name, you can use ActiveSheet.Shapes
to iterate through all the shapes on the worksheet.
Getting to the actual text in the TextBox is not very straightforward in VBA. The following code iterates through all the Shape
objects on the active worksheet:
Dim shp As Shape
Dim myText As String
For Each shp In ActiveSheet.Shapes
myText = shp.TextFrame2.TextRange.Characters.Text
Next
Though I see that you are working in C#, so the above code will be a little different, but it at least gives you the object model to get to the text inside the TextBox
.