The image shows a string attached directly to the slider of the zipper and not to its pull tab, which is seemingly missing. The pull tab is a tirette; the string is used as a replacement for it in the image:

Cordon(/sangle/languette) servant de tirette.

Or the string is "connected" to the pull tab by its nature:

Cordon(/sangle/languette) de [of the] tirette.

If the string was attached to the pull tab per se, then it’s like an extension:

Rallonge à/de tirette.

Metonymic use might simply be tirette; some may reference its material (…en corde, en tissu).

In the end the tirette will go a long way…

Another suggestion based on Zéhontée Bonteuse‘s great answer:

Rallonge à/de tirette en forme de boucle.

It is longer but help communicate the shape of the rallonge.