Login to 123-reg.co.uk or your domain registrar they are pretty similar.
You can check your domain registrar FAQ section likely for a tutorial on how to URL forward.
Head to Domains then you’ll see all your available domains
Select your domain then hit Manage
Once the main management page shows hit on Web Forwarding
Then you’ll see this, hit the add button.
Type in your 000webhostapp URL into the destination box.
Some registrars will offer masking which means once it redirects to your 000webhostapp URL it will retain your custom domain by ways of masking it instead of displaying your 000webhostapp URL.
Either way is acceptable, leaving your 000webhostapp URL (Permanent 301) not (Forward with masking/ framed web forwarding) will ensure the HTTPS/SSL lock is displayed.