How to create events on Design time on C#?

Depends: if you are talking about adding an event to your class that users of your class can handle, then it's pretty easy - just add the code:

/// <summary>
/// Event to indicate [Description]
/// </summary>
public event EventHandler Name;
/// <summary>
/// Called to signal to subscribers that [Description] occured
/// </summary>
/// <param name="e"></param>
protected virtual void OnName(EventArgs e)
    {
    EventHandler eh = Name;
    if (eh != null)
        {
        eh(this, e);
        }
    }

If you are talking about adding a handler to an existing event, then just highlight the object in the designer and:
Look at the Properties pane.
Press the Events button - it looks like a lightening bolt.
Find the event you want to handle.
Double click it.

That will add the handler, and a bare-bones handler routine for you to fill in.

If you are often going to create events it might be worth your looking at this: A simple code snippet to add an event[^]
It makes it a lot easier to generate the above code!