Although there is a lot of confusion in your question regarding microstrip line and stripline (they are different) . You can check for epsilon option in modal solution in HFSS for dielectric constant value.

Actually it is a microstrip antenna. I need to test the unknown dielectric constant of a material. Material is placed on top of microstrip. How can I determine the permittivity of this material now?

hello Usama,

check the attached paper you find useful informations

B.regard