The InsertLineL() function is for adding a dialog line as in the DLG_LINE definition section in the .rss file. So you need to supply all the same resource information.
The Information dialog is the only standard dialog with a plain line of text on it. However, the control it uses for displaying the text is private. Judging by all the other dialogs on the system, any text is meant to fit into the title line and the rest of the dialog is for controls only, so what you seem to be wanting to do is against the UI standard. So you are probably better off just putting your text in the title line instead of a text line.