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I strongly suggest creating a page for your subdomain then you can embed the form using the embed code listed here: https://www.jotform.com/help/67-Which-form-embed-code-should-I-use
I strongly advise using the iFrame embed code.
If what you would like to do is redirect your subdomain link to your JotForm form instead of embedding, that should be possible by following this guide: https://www.jotform.com/help/144-How-to-use-Custom-Domains-with-Jotform. However, it does NOT support SSL (HTTPS). I strongly suggest embedding the form instead.
I hope that helps.