The easiest way I know of is to simply enter two slashes as text (not within the lyrics). You can put them wherever you want, among the lyrics or within the staff. Probably, you will want to choose a normal (non-italic) or bold font of a slightly larger size than usual. This is accomplished under File > Page Setup >Fonts to specify a user-defined font. Then, when you place the text, choose your font.

I have also seen pre-drawn marks of the kind you require (more artistic than two simple slashes). I'm not sure, but believe they can be found in the "Boxmarks2" font accessible from the NWC user links. As before, you would need to define it as a user font.