Here is how to Create an Instance for a Subdomain
1 - Within your Impreza Panel, click on "WordPress Management" on the top right of the page
2- Here, click on "Add New" to create a New Instance
3- Now here just click on your current WordPress Hosting
4- Now in "New Installation", type the Name of your Website, then your Subdomain
5- Alright, that's it, now you only need to wait until the Installation be Completed