This is an example of an ExternalLookup configuration that uses SPDataSource and AutoCompleteRenderMode.

{
  "DataSourceType": "SP",
  "DataSourceConfiguration": {
    "SourceList": "{SiteUrl}/DepartmentCodebook"
  },
  "IndexValue": {
    "Expression": "{ID}"
  },
  "TextValue": {
    "Expression": "{Title}"
  },
  "DisplayValue": {
    "Expression": "{Title} ({ID})"
  },
  "SearchElements": ["Title"],
  "RenderMode": "AutoComplete",
  "EmitEmptyItem": true
}
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