Default CssClass on Custom Control

[Memnoch1207]

I am creating a custom ASP.NET textbox control and I am trying to set the default CssClass, but I can't figure out how.
I have added attributes to the controls (OnBlur and OnFocus) those work fine changing the cssClass, but when the page initially loads the textbox doesn't have a default cssClass.

Here's my code

Imports System
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.ComponentModel
 
Namespace JHATextBox
 
    Public Class JHATextBox
        Inherits TextBox
 
        'This should be the default cssClass when the page loads.
        Private _cssClass As String = "formBox"
 
        Protected Overrides Sub Render(ByVal writer As HtmlTextWriter)
            Dim strScript As String = ""
 
            If Page.IsClientScriptBlockRegistered("ClientScript") = False Then
                strScript &= "<script language=""javascript"" type=""text/javascript"">" & vbCrLf
                strScript &= vbTab & "function activeField(ptr){" & vbCrLf
                strScript &= vbTab & vbTab & "if((ptr.disabled != true) && (ptr.readOnly != true)) {" & vbCrLf
                strScript &= vbTab & vbTab & vbTab & "ptr.className = 'formBoxSelected';" & vbCrLf
                strScript &= vbTab & vbTab & "}" & vbCrLf
                strScript &= vbTab & "}" & vbCrLf
                strScript &= vbTab & "function inactiveField(ptr) {" & vbCrLf
                strScript &= vbTab & vbTab & "if((ptr.disabled != true) && (ptr.readOnly != true)) {" & vbCrLf
                strScript &= vbTab & vbTab & vbTab & "ptr.className = 'formBox';" & vbCrLf
                strScript &= vbTab & vbTab & "}" & vbCrLf
                strScript &= vbTab & "}" & vbCrLf
                strScript &= "</script>" & vbCrLf
 
                Page.RegisterStartupScript("ClientScript", strScript)
            End If
 
            writer.AddAttribute("onBlur", "void inactiveField(this);")
            writer.AddAttribute("onFocus", "void activeField(this);")
            writer.RenderBeginTag("input")
            writer.RenderEndTag()
 
        End Sub
 
        Public Overrides Property CssClass() As String
            Get
                Return _cssClass
            End Get
            Set(ByVal Value As String)
                _cssClass = Value
            End Set
        End Property
 
    End Class
 
End Namespace

If I just type formBox as the CssClass into the controls properties, it works fine, but I can't set it in the code above for some reason.