Are you sure you haven't chosen the "Responsive Form" option at the beginning? There indeed is no such option.
I see no attachment under your question, thus I can't tell for sure, that this is the reason
Do not confuse Responsive forms with Mobile forms. Although they are responsive thus will perfectly work on mobile devices their goal is much wider
You can find my review of the new Responsive forms here.
You can also read about them more here: https://community.nintex.com/blogs/NintexProductBlog/2017/08/14/whats-new-for-forms-for-office-365-july-2017
So if you want to have custom JS, now you have to delete your Responsive form (click the "delete" icon in the Design tab) and start creating Nintex Form once again, this time choose "Standard" form.
Got it! Thanks Tomasz for the clarification on the differences between the form types.
If you find my answer as correct, please mark it as the solution for your question.
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