How can I add product to LineItems portal under the Invoice Layout? I am using New Window to search a Product (portal) then when select it opens New Window (based: on LineItems)and i can add quantity to that product?

I need to get the invoiceid and productid to New Window (LineItems) in order to add record to that LineItems based on a Invoice Layout.

I hope there is someone that could help me with I'm stuck in this ,for 2 weeks now,,please need some help!asap 

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The above thread was all about pickers. I posted a demo of the technique we use, but there are a few approaches. In short, show a card window with a list or portal of choices. The window is typically a different context from the parent window which is why card windows fit the need. User selects one or more items. The picker simply captures the primary keys of the items selected. The IDs are passed to a script that creates new records and sets those ids along with the required foreign parent key. Voila, you have child records. 
 

google FileMaker pickers for more techniques.