Your description confuses me. On a Product form you have a portal of products from the same Supplier? Shouldn't that portal be on a Supplier form?
Do you want to click the portal row and have that become the parent record? If so, use the go to related record command.
It sounds like you are setting up a scrolling list (hence the portal) of items you can select for your current record.
Put a button in the portal row and set it to run a script like this...
#PortalTable is Table Occurrence name used in portal
#MainTable is table occurrence name used for your layout
Set Field [MainTable::ItemID; PortalTable::ItemID]
If ItemID uniquely identifies the item, there are several options for using a relationship to reference all the fields in the portal row once this script has been performed to select an item.