Looks like there is another way - try this.

Admin > Advanced > Other settings (Application Configuration).  Find and tick the options for Suspend Host Control

The wording says

"Prevent the host from controlling the guest's keyboard and mouse upon initial connection."

If you don't see the Advanced menu you need to install a extension called 'Advanced Configuration Editor' by 'ConnectWise Labs'

You need to reinstall the client on the machine you use to join the session (your machine) not their machine

For those stumbling across this issue

Edit App.Config inside App_ClientConfig

Add this to the ScreenConnect.UserInterfaceSettings section

setting name="SuspendControlAtStartup" serializeAs="String"

True

I'm unsure if you need to reinstall the client app or the host app for this to work