Renaming SharePoint URL

Hello, 

I have been trying to rename my SharePoint URL from e.g. companyname625.sharepoint.com to

companyname.sharepoint.com.

I have checked the following article (https://learn.microsoft.com/en-us/sharepoint/change-your-sharepoint-domain-name) on how this can be done, however, i am having issues with step 3 where i have to add a custom domain. In the article they are adding fabrikam domain but i am not sure why it doesn't have any extensions such as .com, or .co.uk ... i have a few domains in my portal such as companyname.co.uk but when i am trying to add a custom domain such as companyname.co.uk.onmicrosoft.com or companyname.onmicrosoft.com it simply doesn't work. 

Any help will be appreciated, Thank you