Check string for values in other table

Say you have your posts in a table, then you can add a column with this formula:

List.First(List.Select(ListOfPotentialCompanies, (x) => Text.Contains([Post], x))) 

You can also paste this code into the advanced editor to see how this works:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8kjMyclX8M1MLsovzk8rUYrViVbyVMjJL0tVCMgvTy1ScPJUio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Post = _t]),
    ListOfPotentialCompanies = {"Microsoft"},
    #"Added Custom" = Table.AddColumn(Source, "Custom", each List.First(List.Select(ListOfPotentialCompanies, (x) => Text.Contains([Post], x))))
in
    #"Added Custom" 

As you'll see, only the first value from the possible list of matches is selected, but you can omit that part of the formula if you want to retrieve all matches instead.