To Add a Function To a new Button on your Form: (and avoid using macro to call function)
After you created your Function (Function MyFunctionName()) and you are in form design view:
Add a new button (I don't think you can reassign an old button - not sure though).
When the button Wizard window opens up click Cancel.
Go to the Button properties Event Tab - On Click - field.
At that fields drop down menu select: Event Procedure.
Now click on button beside drop down menu that has ... in it and you will be taken to a new Private Sub in the forms Visual Basic window.
In that Private Sub type: Call MyFunctionName
It should look something like this:
Private Sub Command23_Click()
Call MyFunctionName
End Sub
Then just save it.