The answer (or at least the ingredients for an answer) can be found in the WordReference dictionary. See definition B3 for tell (give instructions or direct somebody to do something with authority), and definition A1 for ask (make a request or demand for something to somebody).
So you see that "tell" must mean that there is some presumption of authority and/or an expectation that the order will be fulfilled, while "ask" might mean this or it might simply refer to a request for a favor.
No, you would use 'tell' in that sentence: "Go and tell senior that I need him."
You would use 'ask' in this sort of sentence: "Go and ask senior to come to help me."
Then your friend will ask a question, something like "Will you come to help dumbfounded?"
(We wouldn't use 'senior' by itself in this way. If you are interested, you should start another thread to ask what you should use. Explain who 'senior' is, and ask people what to use there.)