What do you want to populate?
You select the CustomerID and then you will have two sets of data showing on your Main Form.
One set is the fields displayed from your tblOrderHeader which will be OrderID (autonumber) OrderDate - you enter this. You can set the form control to use =Today() as default,
The Other is data from the tblCustomer which won't be in the tblOrderHeader.
You can use another SubForm - just a small one - to display this data and link it with CustomerID to CustomerID. This will be "Read Only" and you should make the text box controls properties to not allow any edits etc.