Looking at one of the publically available examples, it seems like this is achievable Mati. I started on spectrumspatial.com and selected the Public sector demo.
- I there added an annotation (in my case a circle of 200 m radius).
- In the layer control, I clicked the 3-dot symbol and selected Summarize
- In the left side panel I choose to sumarize from the Cadastre layer
- For the resulting selection of 65 properties SSA displays min, max, average and sum for all numeric attributes
Also see my screenshots to illustrate this demo example. Note that I did not build this, so I don't personally have the underlying code.