This is called a conditional value list and you can learn more about how to set them up by searching that term out in the Knowledge Base which you can find in the Support menu at the top of your screen.
In your case, you'll need a relationship between Purchase_Order and Product that looks something like this:
Purchase_Order::CompanyID = ProductsByCompany::CompanyID // ProductsByCompany is a table occurrence of Products
Now you can specify values for a value list from ProductsByCompany and use the "show only related values starting from Purchase_Order" to get a list of only those products that have the same CompanyID number in Products.
If "table occurrence" is a new term, see this thread to learn more: Tutorial: What are Table Occurrences?