First you need to set the material setting of the object that you want to be reflective. You must find the material that is applied to the object in the browser tab. You can search for it in the browser tab or you can press Ctrl+Shift then left-click on the object to select the material that is currently applied to it.
In the properties panel increase the Reflectivity and Smoothness properties to 1.00. The Sky-box will be reflected on the material. In order to also reflect objects in the scene we will need to create a Reflection Probe, to do this select the object, Right Click > Create > Lights > Reflection Probe, this can also be done in the Scenes or Developer tree. This will add a reflection probe to you object. This can also be created via the Developer tree by selecting the object there and following the same path to create the Reflection Probe.
Move the probe so that the it is at the center of the object. To see a visual indication of the probes location you are able to to enable the helper which is located in the tool bar.
Move and scale the probe so that the object fits inside of it. In the probes properties you can also change the shape of the probe to get a better reflection.