Make window not activatable by user, using window handle c#

I'm trying to make an external program window not activatable by user. I'm having the handle of that window, but not sure what API to call with WS_EX_NOACTIVATE or similar.

I've found following code sample, but it overrides within project window.

private const int WS_EX_NOACTIVATE = 0x08000000;
protected override CreateParams CreateParams
{
    get
    {
        var createParams = base.CreateParams;

        createParams.ExStyle |= WS_EX_NOACTIVATE;
        return createParams;
    }
}

How can I call external window to make that window cannot get keyboard focus even if user clicks the taskbar button, using windows + tab window switching, or after closing another program etc.?

I just want the user to not allow to send keyboard events to the desired application. (For example, open Notepad window which I'm having the handle)

Any help would be highly appreciated.