Webtool to design guide RNA (gRNA) for use with CRISPR-AsCpf1?
Most online tools will only help you to design gRNAs for Cas9 because it is the one that is most commonly used. I found an online suite called RGEN tools that also has an option for AsCpf1. 

However, if you don't find it satisfactory then my suggestion is that you either design your gRNA manually or write your own script (which won't be that complex).