public bool SelectValue(
    string exportValue
)

Public Function SelectValue(
    exportValue As String
) As Boolean

using DevExpress.Pdf;
using System.Drawing;
//...

static void Main(string[] args)
{
    using (PdfDocumentProcessor processor = new PdfDocumentProcessor())
    {
        // Create an empty document.
        processor.CreateEmptyDocument("..\\..\\Result.pdf");

        // Create graphics and draw a combo box field.
        using (PdfGraphics graphics = processor.CreateGraphicsPageSystem())
        {
            DrawComboBoxField(graphics);

            // Render a page with graphics.
            processor.RenderNewPage(PdfPaperSize.Letter, graphics);
        }
    }
}

static void DrawComboBoxField(PdfGraphics graphics)
{
    // Create a combo box field
    PdfGraphicsAcroFormComboBoxField comboBox =
        new PdfGraphicsAcroFormComboBoxField("combo Box", new RectangleF(20, 20, 100, 20));

    // Add values to the combo box.
    comboBox.AddValue("Red");
    comboBox.AddValue("Yellow");
    comboBox.AddValue("Green");
    comboBox.AddValue("Blue");

    // Specify combo box selected value, text alignment, and appearance.
    comboBox.SelectValue("Red");
    comboBox.TextAlignment = PdfAcroFormStringAlignment.Far;
    comboBox.Appearance.BackgroundColor = Color.Beige;
    comboBox.Appearance.FontSize = 14;

    // Add the field to the document.
    graphics.AddFormField(comboBox);
}

Imports DevExpress.Pdf
Imports System.Drawing
'...

Shared Sub Main(ByVal args() As String)
    Using processor As New PdfDocumentProcessor()

        ' Create an empty document.
        processor.CreateEmptyDocument("..\..\Result.pdf")

        ' Create graphics and draw a combo box field.
        Using graphics As PdfGraphics = processor.CreateGraphicsPageSystem()
            DrawComboBoxField(graphics)

            ' Render a page with graphics.
            processor.RenderNewPage(PdfPaperSize.Letter, graphics)
        End Using
    End Using
End Sub

Private Shared Sub DrawComboBoxField(ByVal graphics As PdfGraphics)

    ' Create a combo box field
    Dim comboBox As New PdfGraphicsAcroFormComboBoxField("combo Box", New RectangleF(20, 20, 100, 20))

    ' Add values to the combo box.
    comboBox.AddValue("Red")
    comboBox.AddValue("Yellow")
    comboBox.AddValue("Green")
    comboBox.AddValue("Blue")

    ' Specify combo box selected value, text alignment, and appearance.
    comboBox.SelectValue("Red")
    comboBox.TextAlignment = PdfAcroFormStringAlignment.Far
    comboBox.Appearance.BackgroundColor = Color.Beige
    comboBox.Appearance.FontSize = 14

    ' Add the field to the document.
    graphics.AddFormField(comboBox)
End Sub

// Specify combo box selected value, text allignment, and appearance.
comboBox.SelectValue("Red");
comboBox.TextAlignment = PdfAcroFormStringAlignment.Far;

' Specify combo box selected value, text allignment, and appearance.
comboBox.SelectValue("Red")
comboBox.TextAlignment = PdfAcroFormStringAlignment.Far