public PdfGraphicsAcroFormFieldAppearance Appearance { get; set; }

Public Property Appearance As PdfGraphicsAcroFormFieldAppearance

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 and draw a text box field.
        using (PdfGraphics graphics = processor.CreateGraphicsPageSystem())
        {
            DrawTextBoxField(graphics);

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

static void DrawTextBoxField(PdfGraphics graphics)
{
    // Create a text box field and specify its location.
    PdfGraphicsAcroFormTextBoxField textBox =
        new PdfGraphicsAcroFormTextBoxField("text box", new RectangleF(0, 10, 200, 30));

    // Specify text box properties.
    textBox.Text = "Text Box";
    textBox.TextAlignment = PdfAcroFormStringAlignment.Near;
    textBox.Appearance.FontSize = 12;
    textBox.Appearance.BackgroundColor = Color.AliceBlue;

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

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 and draw a text box field.
        Using graphics As PdfGraphics = processor.CreateGraphicsPageSystem()
            DrawTextBoxField(graphics)

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

Private Shared Sub DrawTextBoxField(ByVal graphics As PdfGraphics)

    ' Create a text box field and specify its location.
    Dim textBox As New PdfGraphicsAcroFormTextBoxField("text box", New RectangleF(0, 10, 200, 30))

    ' Specify text box properties.
    textBox.Text = "Text Box"
    textBox.TextAlignment = PdfAcroFormStringAlignment.Near
    textBox.Appearance.FontSize = 12
    textBox.Appearance.BackgroundColor = Color.AliceBlue

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