readCSV[file_] := Import[file, "CSV"]

NotebookOpen @ FileNameJoin @
  { $InstallationDirectory, "SystemFiles", "FrontEnd", "SystemResources"
  , "FunctionalFrequency", "specialArgFunctions.tr"
  }

"Import"->{2, "ImportFormats_Names.trie"},

"readCSV"->{2},

addCompletion = FE`Evaluate[FEPrivate`AddSpecialArgCompletion[#]] &

addCompletion["readCSV" -> {2}]

"Style"
"ScreenStyleEnvironment"
"PrintingStyleEnvironment"
"MenuListStyles"
"MenuListScreenStyleEnvironments"
"MenuListPrintingStyleEnvironments"
"codingNoteFontCom"
"ExternalDataCharacterEncoding"
"MenuListPalettesMenu"
"MenuListStyleDefinitions"
"ConvertersPath"
"MenuListDisplayAsFormatTypes"
"MenuListConvertFormatTypes"
"MenuListCellTags"
"MenuListWindows"
"MenuListHelpWindows"
"MenuListPackageWindows"
"MenuListPaletteWindows"
"MenuListPlayerWindows"
"MenuListStylesheetWindows"
"MenuListTextWindows"
"MenuListFonts"
"MenuListGlobalEvaluators"
"MenuListNotebookEvaluators"
"MenuListStartEvaluators"
"MenuListQuitEvaluators"
"MenuListNotebooksMenu"

addCompletion["myImport" -> {2, $ImportFormats}]

AddCodeCompletion = ResourceFunction["AddCodeCompletion"]
AddCodeCompletion["readCSV"]["AbsoluteFileName"]