Populating textbox from external class or include

[ar]

If I had a form with one textbox on it and I wanted to populate that textbox
on page load, how could I do that from a global procedure?

I tried creating a separate class file with the following code and then
called it from my aspx page:

Public Class Class1
Public Shared Function setbox(ByVal textboxx As Object)
Dim mycontrol As Control = FindControl("txtName")
CType(mycontrol, TextBox).Text = "TESTTTT"
End Function
End Class

I just get an error that FindControl is not declared. (txtName will
eventually be the name of a control that is pulled from a database..)
Basically I just want to be able to re-use the same code to populate my forms
with values from a database. Or if I can't populate a form control from an
external class file, can I do some kind of #include to make it work?

(I have a function in my codebehind that will pull all of my control names
for a form from a database and populate each one with data from the
database.....I just don't know how to move that function out of my codebehind
into a global area so I don't have to paste the long function into each
codebehind page.)

Thanks in advance for any advice.