Existing Dropdown from a Lookup list to another List.

1. Select the DropDown
2. Change the Item property Datasource:
   //Choices([@’DataSource’].FieldName)
   Distinct(‘DataSource’, FieldName)
3. Change the DisplayFields property to:
   From: [“Value”] To: ThisItem.’ThisListFieldName’
4. Select the whole datacard
   Change Update: ThisListFieldName.Selected.Result

Allow Empty Selection for dropdown list in a canvas app
Have the dropdown box selected -> advanced->Data ->AllowEmptySelection = true