Your first button handler should check the result of ShowDialog():
Private Sub Button4_Click(sender As Object, e As EventArgs) Handles b_Script.Click
Dim BrowseFolder As New FolderBrowserDialog
If BrowseFolder.ShowDialog() = Windows.Forms.DialogResult.OK Then
tbScript.Text = BrowseFolder.SelectedPath
End If
End Sub
Your second handler should use Path.Combine():
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim Path As String = System.IO.Path.Combine(tbScript.Text, "Client_Perfmon.ps1")
RunScript(Path)
End Sub
*It should probably check to make sure the path actually exists as well!
What does RunScript() do?
Sometimes applications require that paths with spaces be enclosed in quotation marks:
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim Path As String = Chr(34) & System.IO.Path.Combine(tbScript.Text, "Client_Perfmon.ps1") & Chr(34)
RunScript(Path)
End Sub