You can connect a custom domain by following these steps:
Log in to your Writing.io profile.
On your dashboard, click the “Settings” button towards the top of the page.
Scroll down to “Domain Settings.”
Enter your custom domain.
Click “Continue.”
You will be shown a CNAME record. Login to your domain name registrar (the service you used to register your domain name) and add this CNAME record to your domain name.
Once you have added the CNAME record with your registrar, your domain should point to your Writing.io site within 24 hours.