For security reasons, you should own a domain to use it with Hostinger services. If your domain is located in a different Hostinger account - all you need to do is add a TXT record to verify the domain.
Step 1 - Receive a TXT record
Copy the TXT record you received. The screen you receive TXT record might look like this:
Step 2 - Add a TXT record
Go to your domain’s DNS zone and add a TXT record with the following value:
the value which you copied on a previous step
If your domain is pointing elsewhere by NS records, your DNS Zone management is moved to the provider you pointed the domain to and should be managed from there
When you update your domain's DNS zone, you will need to wait up to 24 hours until changes are fully propagated.
Step 3 - Add the website to your hosting plan
After propagation is completed, you will be able to add the website to your hosting plan without any problems.