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https://nishantrana.me/2020/05/26/planning-to-show-a-custom-multi-select-lookup-use-the-xrm-utility-lookupobjects-to-show-a-multi-select-lookup-control-in-dynamics-365/
Planning to show a custom multi-select lookup. Use the Xrm.Utility.lookupObjects to show a multi-select lookup control in Dynamics 365