You can publish the project through the subdomain that was created in the Multipass Token (see multi-pass example). If you need to connect a custom domain you just need a to configure NGINX or any other similar service as a reverse proxy for HTTP with support for modifying request headers.
You just need to modify the Host header.
Nginx Example Implementation
location = / {
proxy_pass http://34.237.47.210; //Cloud IP
proxy_set_header Host xxx.preview-domain.example; // your subdomain
}