Welcome to KH. Is this for the I cord edging? It’s oddly worded. You’ll do the cast on then attach the yarn in the center of the cast on edge. I found a video that might help. We can’t see (image 01) as this is a paid pattern. You might copy it and post it here.
I have knit this vest so can help you! The back neck is a provisional cast on and you’ll unravel half of this so that your stitches are on the needle.
Working from the wrong side, you’ll start in the centre of the back neck. The three stitches that you cast on will start the applied i-cord. These three stitches will form the base of the i-cord when you come to work the other side of the neck.