Based on [this answer by Carl Woll][1], the syntax should be:

    Cell[StyleData["Input", StyleDefinitions -> StyleData["Code"]]]

Setting the option programmatically:

    SetOptions[
        EvaluationNotebook[],
        StyleDefinitions -> Notebook[
            {
            Cell[StyleData[StyleDefinitions -> "Default.nb"]],
            Cell[StyleData["Input",StyleDefinitions->StyleData["Code"]]]
            }
        ]
    ]

Checking:

[![screenshot][2]][2]

As one can see, the input cell is formatted like `"Code"` cell, while in its definition it still has style `"Input"`. Done!


  [1]: https://mathematica.stackexchange.com/a/199438/280
  [2]: https://i.sstatic.net/JFVw7.png