Comparing two tables with only first value of column

pls try

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUQrIslLwzUzXAdHOpcUl+bmOBQU5SrE60UpGUGk0YWOosGtFajJYwARJQAfZPMecnMzEvORU5/zcXKXYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [No = _t, Project = _t]),
    lst = TblPjID[ActivityID],
   f=(x)=>[a=Table.PositionOf(TblPjID,x,Occurrence.Last,g),b= {"NOMATCH",lst{a}}{ Byte.From(a>-1)}][b],
   g=(x,y)=>   Text.Split( y[Project],","){0}=x[Project],
     to = Table.AddColumn(Source,"ActivityID",f)
     in
     to

Hi Ekaterina_, check this:

Result

let
    Table1 = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUQrIslLwzUzXAdHOpcUl+bmOBQU5SrE60UpGUGk0YWOosGtFajJYwARJQAfZPMecnMzEvORU5/zcXKXYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"No." = _t, Project = _t]),
    Table2 = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCsiyUnCtSE1W0lEKDg/QBTNjdSDivpnpQGEfnwBdEAsm6lxaXJKf61hQkKMA1YQQgStyzMnJTMxLTnXOz80FqnIM1kURiY0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Project = _t, ActivityID = _t]),
    T1_Ad_Helper = Table.AddColumn(Table1, "Helper", each Text.Trim(Text.BetweenDelimiters([Project], ":", ",", 0, 0)), type text),
    T2_ExtractedProject = Table.TransformColumns(Table2, {{"Project", each Text.Trim(Text.AfterDelimiter(_, ":")), type text}}),
    MergedQueries = Table.NestedJoin(T1_Ad_Helper, {"Helper"}, T2_ExtractedProject, {"Project"}, "T2", JoinKind.LeftOuter),
    ExpandedT2 = Table.ExpandTableColumn(MergedQueries, "T2", {"ActivityID"}, {"ActivityID"}),
    RemovedColumns = Table.RemoveColumns(ExpandedT2,{"Helper"})
in
    RemovedColumns