To create a mirror material, follow these steps:
Navigate to Global Preferences and Settings and select Materials.
Open the User Created Materials dropdown menu and select Add.
In the Material Editor window, specify a name and set the following properties:
Color: White
Specular: 1
Roughness: 0
Metalness: 1
Refractive Index: 1
Transmission: 0
Opacity: 1
Emissivity: 0
Select Create New Material to add this material
In order to APPLY this material onto an element, follow these steps:
Select the element on which you want to apply the material, and navigate to the Materials section in the properties panel
Open the material drop down menu.
To access the materials created by the user, select the User Created tab.
Click on the desired material thumbnail to apply it on the element.