Define and invoke function within query

That would be done like so:

let

	AssessmentsInfoFunction = (AreaAPI as text) as list =>
	let
		Source = Json.Document(Web.Contents(“https://abc.xxx.com”,
		[
		RelativePath= “/api/v1/gateway/platform/”&AreaAPI&”/assessmentcriteria”
		]
		))
	in
		Source,

	Source = {ClientDetails[RootAreaGuid]},
	#"ListOfLists" = List.Combine(Source),
	#"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
	#"Renamed Columns" = Table.RenameColumns(#"Converted to Table",{{"Column1", "AreaGuidsForFunction"}}),
	#"Expanded AreaGuidsForFunction" = Table.ExpandListColumn(#"Renamed Columns", "AreaGuidsForFunction"),
	#"Added Custom1" = Table.AddColumn(#"Expanded AreaGuidsForFunction", "Custom", each AssessmentsInfoFunction([AreaGuidsForFunction])),
	#"Merged Queries" = Table.NestedJoin(#"Added Custom1",{"AreaGuidsForFunction"},ClientDetails,{"RootAreaGuid"},"ClientDetails",JoinKind.LeftOuter),
	#"Expanded ClientDetails" = Table.ExpandTableColumn(#"Merged Queries", "ClientDetails", {"ClientName"}, {"ClientName"})
in
	#"Expanded ClientDetails"