Line Loop to pull data from Purchase Order and place in UDO

Hi,
I'm wanting to create a button on the UDO i've created that pulls through Purchase Order info (docnum, cardcode, cardname, Itemcode, description, Quantity). It needs to search through all Open POs and pull the relevent lines through where the line field "container" matches the container field in the header of the UDO. Can someone offer some guidance on the line loop process for this please?