Rick's suggestion, in addition to being faster, will also work with subclasses of textbox.

You can also try the one line Linq version (untested):

loHtmlForm.Controls.OfType(Of System.Web.UI.WebControls.TextBox)() _
.ToList() _
.ForEach(Function(loTextbox) loTextbox.Attributes.Add("onkeypress", "FormKeyPress(event)"))