How do i block a piece that is still on the needles?

The shawl pattern that i am working on calls for a lace border in a “lengthwise edging”. Before starting the edging, while leaving the piece on the needles, it instructs to block the piece. How do i do that when the piece stretched out is longer than the needles??

what about putting the working stitches on waste yarn and secure your stitches that way before blocking?

That’s what I said, only the computer ate my reply!!!

Actually, I suggested crochet thread or plain dental floss.