If your hosting company provides cPanel or another branded server admin area, you should look for the "Aliases" option in order to add a second domain.
Here's a video about how to configure Aliases in cPanel:
Important: When creating the Alias, ensure that the root folder is the same as your Wordpress installation. Additionally, in some server environments "Aliases" may go by a different name and may require a different type of configuration. Other names include Domain Alias, Domain Mapping, Multi-domain, Virtual Hosting, etc. The point is to ensure that the domain uses the same root folder as your WordPress install.