Parked domain doesn't show new domain name

Maybe I’m stupid. But i need help.

I bought a 3th part domain. I configured this domains ns record to use 000webhost dns servers. ok I parked my 3th party domain in the settings of my site on 000webhost.

result: the domain works. But what I paid for doesn’t work.

When I type my domains url in the url bar of my browser, it connects to my site, but it shows the old temporary 000webhost url afterwards.
I don’t want this, it should give me my domain name after opening the site in the url bar. That’s why we want a domain no?

It is only redirecting it seems, but I didn’t use a redirect record or something.
How to keep it showing my new 3th party domain name after opening the site?

please help somebody…it can’t be difficult.

What domains are we talking about?
What script are you running on your site?
If WordPress:

Check this tutorial and let me know how it went for you