How To Encrypt Any Given String

I made this program to encrypt any given string, I am getting an error that prevents this from being finished.

Public Class Form1
Dim Holder As string
Dim Pass = "Za029fjao"
Dim Helper As String
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Holder = TextBox1.Text
Helper = Holder + Holder * Pass
Textbox2.text = Helper
End Sub
End Class

All I want to do is multiply 2 strings (is that possible?). The error says that string cannot be converted into double...