How to add a fixed position for mobile

I received an email from linkedin on gmail that had a fixed position at the bottom which showed up only on the mobile which blew my mind. Unfortunately, gmail striped out all the media query code, so I couldn't really check the coding. So I googled it and found an two years old article from Campaign Monitor, but when I tested it out, it didn't work.
https://www.campaignmonitor.com/blog/email-marketing/2013/09/how-to-add-a-fixed-position-cta-to-your-email-newsletters/

Here is the linkedin screen shot:
https://www.dropbox.com/s/dul2z19awrrp23v/linkedin.PNG?dl=0

Any suggestion on how to use fixed positioning on mobile email?

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