As mentioned by Noor HIDAYAH Mohd Yunus , you have to know/characterize the parameters you need for your simulation and obviously over the frequency band of your interest. You don't need to define all specifications of SiO2 but, need epsilon r and conductivity over your band of interest for electromagnetic simulations; for absorption rate measurement (SAR type of thing) you also need to define material density.
In assign materials options ,you have a option of having new material to CST library. Just create new material with electrical properties..see fig...hope this helps .