How do I add a design to exact keyline/dieline

Bantei,

As far as I can see, the keyline is custom made to exactly fit the rectangular bowl with its rounded corners obtained by the straight and curved parts at top and bottom, so "the arc option" would be off, because it is a (crude ) circular shape; and the pink area outside the keyline made to mark the bleed.

I agree with Bobby in using a blend to make the centre line and then (one) Art Brush(es).

In this connexion, I believe it is worth trying to build it on one single Art Brush, more or less as follows, depending on whether the actual design (apart from background colouring) is to A) form a continuous band, with the ends abutted on the rear side, or B) ending on both sides of a gap with only the background colouring where the ends meet, see the crude sample images below:

1) Creating a background colouring extending past the pink bleed area, possibly just white = no ink (not shown below);

2) Creating the centre path of the expanded 1 step Blend (not shown below) and hiding/deleting the original top and bottom paths, extending:

2A) Past the keyline and including the bleed at the ends, in other words adding the width of the bleed to the straight part at each end (you can use a guide line and/or Smart Gudides),

2B) As far as the keyline at the ends, in other words no addition;

3) Creating all the artwork in the actual design on top of a transparent horizontal rectangle having the exact length of the keyline from 2) and the relevant height, everything turned into an Art Brush (shown below with gradient and text representing the actual artwork, the dashed line representing the bounds, and the red dashed line representing the midline corresponding to the centre path from 2), with:

3A) The end parts of the actual design fitting each other at the end parts of the keyline (shown as full red lines) with extensions covering the bleed, so the Art Brush spans the length of the centre path past the keyline from 2A), shown in two versions with different heights of the artwork;

3B) The actual design ending half the gap width within the end parts of the keyline (shown as full red lines), with only the transparent horizontal rectangle extending to the full length, so the Art Brush spans the length of the centre path from keyline to keyline from 2B);

4) Applying the Art Brush from 3A) to the centre path from 2A, or applying the Art Brush from 3B) to the centre path from 2B.

Click to get closer

You can see how it works and adapt if needed.

Always keep backups of the artwork underway to fall back on.

What kind of design you want to create? Is it included object or text also? Can you share any mockup?

If the artwork has to fit that arching shape exactly I'd probably use art brushes to build it. First, I'd create a centerline running through the middle of the arc. That can be done by making a 1-step blend between copies of the top and bottom lines of the arching shape. I'd get the path length of the resulting centerline path in the Document Info panel (Objects has to be selected in the flyout menu). From there the artwork can be built to match that horizontal path length. Since the keyline is a bit wobbly it's likely you'll have to build some overshoot or bleed into the artwork and let it get clipped by the boundary of the keyline.