Hey
From the video I understand you want an extension that auto clicks the size, then the checkboxes, submits, then hit's "S'inscrire" again?
Also, will the extension be ran on a computer?
If so you do not really need an extension, you can use tampermonkey which is used to inject JS scripts into websites.
I can do either a tampermonkey script or an extension, depending on what you prefer.
If you have any questions, please do not hesitate to contact me