You will want to setup a new DNS record with whatever company manages your DNS (probably hostinger). Choose type ‘A Record’ and enter the IP address for Bubble’s public server. Then go to your app Settings -> Domain/Email tab and enter your new domain as [subdomain].[primarydomain].com (no https) and then check the SSL box under SSL encryption further down that page. Note: your plan has to support custom domains and SSL.
https://manual.bubble.is/application-settings/custom-domain-ssl.html#setting-up-a-domain