Apple Support Actions

If you call AppleCare they may request controls of your computer, this is perfectly safe and you do not have to say okay. They will not recommend any antimalware solutions, however the best solution is using your common sense. Macs do not need ANY repeat any third party solutions assuming the user uses some common sense such as never downloading from an untrusted site, not using a torrent, not visiting nefarious site and reading and following the advice in Phony "tech support" / "ransomware" popups and web pages

Mac OS is extremely secure and does not benefit in ANY fashion by adding cleaning, antivirus, or any other third party "maintenance" apps. If simply kept up-to-date and other wise left alone Mac OS tends to run extremely well for years.

I was on my iMac & got this popup message which took over the screen
 & instructed me to click on their pick to prevent my computer from 
self destructing in 6 min or something

That would be a scam. Apple does not use pop ups or call you.

So, since you didn't give out any personal info or credit card numbers, but called Apple instead, you should be fine and Apple will sometimes ask for permission to share your screen as mentioned. Malwarebytes for Mac was developed by a long time contributor here and is quite good at removing adware etc.