Richard... sorry, I'm confused - again  :-)
This is the code I wrote:
Private Sub btnInvoices_Click()
    Dim MyPassword As String
    MyPassword = InputBox("Enter Password", "Password")
    If MyPassword = "mypassword" Then
        DoCmd.OpenForm "frmInvoices"
    Else
        MsgBox "Your request has been canceled or you have entered an invalid password.  Please try again or contact your System Administrator.", vbCritical, "Sorry..."
        Exit Sub
    End If  
End Sub

On other fields there would be an INPUT MASK option under the Data tab of the Property Sheet, but buttons don't have this option.  Is there a set of code I could use instead?

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Reply from Richard Rost:

Sorry, I didn't catch that you're using an actual InputBox function for this. No, there's no way to mask the input in this case. What I have done in the past is to create my own little logon form with a single text box (password) and an OK button. Make it popup and modal, and have the OK button check for the password.