If Chromebook devices are not managed and settings cannot be pushed via Google Admin Console, you will need to use Global App Config Service within the Citrix Cloud portal.
On your Citrix Cloud portal, select the menu located on the top left hand corner of your screen and select "Workspace Configuration".

On the Workspace Configuration page, select "App Configuration tab.

Select "Chrome OS" and search for setting called: "Enable Screen Sharing In Microsoft Teams".

On the right had side, you will see a toggle with an "X". This means the feature is turned off. To turn this on, click on the toggle and it will turn green with a check mark.
You will now need to check the box for "Chrome OS" and make sure the toggle for this is also checked and green.
You will see at the bottom of the screen that you have saved drafts that are not published yet. Save the settings to proceed.

Once this has been done, get user to test and they should then be able to screen sharing using their bring your own device Chromebook.
Problem Cause
Configuration