Make sure you host your domain on Cloudflare, it's simply done by porting your name servers. If you're new to this for your agency, follow these guides
Your own domain (please use your root domain,
not a subdomain)
Assuming your root domain is now on Cloudflare, here's what you need to do and keep in mind:
Your root domain will host the white-labelled OnWhatsApp dashboard
Your subdomain will be used to house your website, for example, https://get.onwhats.app
Your customers will login to their dashboard from https://YOUR-DOMAIN.COM/login
For the above setup, we need to enable page rules on Cloudflare
Login to your Cloudflare account and tap Page Rules
Next, Click Create Page Rule
Key in your domain name, do not put * in the end, that'll make the app misbehave, so you enter just your root domain, for example, onwhats.app/
Click Then the settings are dropdown and select Forwarding URL
Next, select Permanent Redirect (301)
Last but not the least, your destination URL should be a subdomain you set, this is where your landing page is, where your customers discover you.
Your final settings should look something like this, though with your domain settings
Now tap Save. Give it a couple of minutes and you're all set!