Copy namecheap_ddns.sh to /usr/local/bin/namecheap_ddns.sh and make it executable sudo a+x namecheap_ddns.sh.
Add the following to the end of /etc.defaults/ddns_provider.conf.
[Namecheap]
modulepath=/usr/local/bin/namecheap_ddns.sh
queryurl=https://namecheap.com
website=https://namecheap.com