Public Class Form1
    Public Sub Test(ByVal Irgendwas As String)
        MessageBox.Show(Irgendwas)
    End Sub
End Class

Public Class Form2
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Form1.Test("Hallo")
    End Sub
End Class

Gruß
Christoph