Certainly - It is a string expression but you need to concatenate the value from the control(s) on your form into the string.
So if you where to do this a little more "expanded" you could see the expression a little more clearly;
Code:
Private Sub btnEditEvent_Click()
Dim strWhere as String
strWhere = "[EventsID] = " & Me.EventID
Debug.Print "Your String is : " strWhere
DoCmd.OpenForm "frmEvents, acNormal,, strWhere
End Sub
If you open the immediate window in the VBA Editor (Press Ctrl + G) you will see the result of the Debug statement.
This is a great way of handling this type of expression as you can easily see what is being passed to the open form command.
Debugging you code in this way can easily show up errors and what is really happening, as opposed to what you "think" is happening...