How to assign same custom domain to my amplify app and the API gateway that it uses

I have an amplify app and an API gateway that is used by the app. API gateway is associated with a lambda function. I can assign domain to them as follows and it works well:

• App: mydomain.com
• API: api.mydomain.com (note that this is a subdomain of above)

But I want to make them same origin, which means I want to assign domain like this:

• App: mydomain.com
• API: mydomain.com (exactly same as above)

How can I achieve this?

Thanks in advance.