There are a few ways to accomplish this.
Mainly, you are going to want to edit the style of the coverage you are sketching, or create an entirely new coverage style. You can do this using the Style Palette.
Once you have opened the Style Palette, choose the Coverage tab. From there, you can add a new style, and customize the color of the new style to be transparent by browsing the "Fill Style" field and assigning the appropriate alpha value for transparency.
OR, to make it a bit easier, you can simply select an existing coverage style by double-clicking on that coverage in the style palette. From there, browse the "Fill Style" field, then select Materials tab, select Materials/Colors, and select Transparent.
I hope this helps!
Principal Specialist, Infrastructure