Using Dex, how Create multiple desktops on UI 8, Android 16, ZF7

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As title says I'm trying to add multiple desktops supposedly up to four using Samsung Dex on zf7 connected to external monitor. Instructions via a search on Google suggest in 8.5 or 8.0 UIs there is a plus symbol and I cannot find this.
Anybody have suggestions, insights or is this unavailable until 8.5? TiA
Peace