An input line can be added or removed from any chat window. To add it, right-click the window in question, go to Window Settings and choose None or Frame Only from the dropdown box. You only need one input line on the screen, so if you have more than one set of tabs you can remove the extra input lines by choosing Frame and Titlebar from those windows.
DarqUI chat windows don't have titlebars, so that setting is used as a toggle. When you are done setting up, right-click one of the windows that does have an input line and choose Always chat here.