I'm sorry. I was assuming that you were familiar with coding procedures.
When you look at the property window for the AddNew command button (you did create a command button to add a new record?) and click on the line for the click event, click the ellipsis (...) to the right of the entry line, and select code builder. Enter the code between the lines so it looks something like this
private sub command_onClick
[PO Number].SetFocus
end sub