I'm not sure what you tried. Is this not what you want?

    >>> df = pl.DataFrame({"x": ["a", "b", "c"], "y": ["d", "e", "f"]})
    >>> df.with_columns(new=pl.col.x + " text " + pl.col.y)
    shape: (3, 3)
    ┌─────┬─────┬──────────┐
    │ x   ┆ y   ┆ new      │
    │ --- ┆ --- ┆ ---      │
    │ str ┆ str ┆ str      │
    ╞═════╪═════╪══════════╡
    │ a   ┆ d   ┆ a text d │
    │ b   ┆ e   ┆ b text e │
    │ c   ┆ f   ┆ c text f │
    └─────┴─────┴──────────┘