How to share screen using app

I am using Spreed.Me app in next cloud and it is working fine.
I have my next cloud installation on RHEL-7.
I am not able to share screen of my computer. in fact the icon for screen share is missing.
This icon was visible when I access my next cloud using plain http.
But it tells with http screen can not be share.
I switched to https, now the icon itself is not visible.

But I thank the team of for making such a nice app. in-fact whole next cloud team is fantastic.

What kind of ssl certificate do you have? Did you install the extension?

Dear Thomas,
Thank you ! for your reply.
I have not installed any certificate yet. I am just accepting to go ahead for my site and browser is reporting a warning sign always. As i wanted to test my screen cast first I just moved ahead with this. I am using firefox version 54. I am trying with google chrome now but my system is not connected with internet and I am struggling to install the extension you have mentioned in your reply.
does the ssl certification must for this to go ahead for screen cast??

Could not download the extension for google chrome.
I have not installed the webRTC, I have just installed the Spreed App in my next cloud.
App is working fine for video and audio chat.
is webRTC bundled in the nextcloud. I have installed next cloud 12.

I want the screen cast to work.
may I get help from some body what are the things missing from my installations.

Thanks in advance.

In my experience screensharing only works with chrome + extension reliable.

thanks Thomas!
I have actually installed video call app.
I thought both apps and video call app is same.
after reading the detail of both I come to know video call app is better as it reduces the configuration efforts for webRTC.
I also found that screen share only works with https.
I have installed my next cloud on a intranet and I don’t have ssl certificate.
I tried with self signed certificate but in that case both chrome and mozilla shows warning of unsecured connection.
I am unable to use screen share feature of video call app.

I wish if some body can help in this.

use letsencrypt they provide a free ssl cert, you just need to update and they provide the scripts to do it, there is a lot of documentation on it for use with nextcloud.

Dear Friends,
I have installed the certificate and I am using Mozilla firefox 52.04, but still the option of screen share is not coming up.
What more, I can do to get it.

I am able to screen cast now.
Screen cast button enables only if two persons are having a connection.
But now I am facing another issue.
When I screencast from windows7 PC the other client is able to see see updated screen.
but if I share from windows8 system the share screen is freezed and no change is getting reflected at other client display.

for firefox go to about;config
search media.getusermedia.screensharing.allowed_domains
add your domain to the list