Up to you. You can create a new button to test the code and use its Click event. Once you get it working, you can copy or move the code to wherever you need it to be for your users to use.
Ok DBGuy...Im going to apologize upfront...I have no clue what I am doing here....BUT...I did manage to "trial and error" my way through some things and I found a couple interesting points.
I placed the below code into a button named, bt_SendEmail. At first, when the button was clicked, it threw an compile error up complaining abut "
For Each var In .SelectedItems" line. After researching a bit, I found the List box collection is called .ItemsSelected I just found this by pure happenstance and unless you were meaning something totally different than what I am assuming...I guess it was just transposed.
But regardless....after I made the change to your code, I then fired off the button and it worked....sorta. Meaning ...your code did exactly what it was supposed to do...It took the various selected emails that I chose in the List box and produced a Access Message box listing all the addresses chosen. My options on the message box was OK and Close.

So....your code worked...however, after I clicked ok...the code went on to follow through the rest of the command and brought my outlook message window with all the specifics I have in my code. But...sad to say.....no email addresses
So then I commented out your msgbox statement and tried it again. This time it went straight to bringing up my outlook message form with all my specifics....but again...no email addresses.
So again, I apologize...I really don't know what I'm doing. Can you take a look at the code below and tell what I am doing wrong?
oh and one more note.... my MultiList Box is named EmailListBx. One more variable I want to throw in the mix is...right now, my List box only contains 1 column - the email address However, i plan to list the Account Name and First and Last Name of the contact so this means ultimately I will have 4 columns in the list box with the email address being in column 3 AccountName = 0 FirstName =1 LastName = 2 EmailAddress = 3
Thanks again for everything...I certainly appreciate your time and effort.
Private Sub bt_SendEmail_Click()
Dim strEmail As String
Dim var As Variant
With Me.EmailListBx
For Each var In .ItemsSelected
strEmail = strEmail & .ItemData(var) & ";"
Next
End With
'MsgBox strEmail
DoCmd.SendObject acSendNoObject, , , Me.EmailListBx.Value, , , "This is a test message subject", "This is the body of my email message. Hello World.", True
End Sub