Namecheap only supports DNSSEC for the following TLDs:
To add DS records for your domain on Namecheap's web interface, follow these steps:
1. Select the domain
2. Go to "Advanced DNS"
3. Enable DNSSEC
4. Add Key Tag, Algorithm, Digest Type, and Digest (you'll find all of those on your Cloudflare dashboard)
5. Save, and you're done!
For a detailed guide with images, Namecheap has a prepared an article that explains how to manage DNSSEC for domains pointed to "Custom" DNS.