When you click the Start Program button, it uses the SetEnvironmentVariable API function to set an environment variable. It then executes another instance of itself. That program will see the new variable.

' Display all environment variables.
Private Sub Form_Load()
Dim i As Integer
Dim txt As String

    i = 1
    Do
        If Environ(i) = "" Then Exit Do

        txt = txt & Environ(i) & vbCrLf
        i = i + 1
    Loop

    txtVariables.Text = txt
End Sub

Private Sub cmdStartProgram_Click()
Dim file_path As String

    ' Set an environment variable.
    SetEnvironmentVariable txtVariable.Text, txtValue.Text

    ' Run a new instance of this program.
    file_path = App.Path
    If Right$(file_path, 1) <> "\" Then file_path = _
        file_path & "\"
    file_path = file_path & "Project1.exe"
    Shell file_path
End Sub