メニューバーにコントロールを追加しクリックでフォームをさせる

Option Explicit

Sub Auto_Open()

    Dim myBar As CommandBar

    Dim myCtrl As CommandBarControl

    Dim strSheet As String

    Set myBar = CommandBars("Worksheet Menu Bar")

    Set myCtrl = myBar.Controls.Add(Type:=msoControlButton, Temporary:=True)

    With myCtrl

        .Style = msoButtonCaption

        .Caption = "Login"

        .Visible = True

        .OnAction = "dspLogin"

    End With

    strSheet = ActiveSheet.Name

    Sheets(strSheet).Select

    Cells(1, 1).Select

    Set myBar = Nothing

    Set myCtrl = Nothing

End Sub

Sub dspLogin()

    ' ログインフォーム

    LoginForm.Show

    MsgBox LoginForm.HostName

End Sub