I use a mix of corn syrup and water, about 1/3 syrup and 2/3 water, brush it on with about a 1/2 inch brush. Works great, no taste added. Some folks put their fondant on while the cookie is still warm, no adhesive needed. Others use warmed preserves or piping gel.
I use RBC on cookies a lot, but not fondant, but I figure it would work, I keep corn syrup in a squeeze bottle with a small tip, and I just apply it like glue close to the edges so they don't pick up, and it sticks great. A little tip, I roll it very thin, so I use an offset spatula to pick it up and onto the cookie.