[Vlad]

He only wanted a function to put in the text in the chat box and do nothing more, so here you go mate:

local function putInChatBox(name)
  name = type(name) ~= "string" and "" or name
  local e = DEFAULT_CHAT_FRAME.editBox
  ChatEdit_ActivateChat(e)
  e:SetText("/tdps whisper "..name)
end

-- example: putInChatBox("Vladinator")

The first line makes sure the param is a string, if not it uses a empty string so the function still works.