What’s your case study about?

Eigen frequency study:

first use Perfect electric boundary conditions and find out the available frequency in the region of interest. Then use Impedance boundary conditions and search eigen frequency near the earlier results, found in perfect electric boundary conditions.

Frequency domain study:

Use two lumped port in the structure (standard SMA connector geometry you can put). One port will be act as a excitation and another port will be receiver. Put some voltages in excitation port and do not put any voltage in receiver. Use co axial type lumped port. Then found the transfer function (S21 parameter) for different frequency and plot it. Then you will found the resonance frequency.

If you can say clearly about the structure then it will be very good.

Defined your geometry, materials, and BC (fixed constraints + any added mass or spring elements that might apply) in Solid you run an eigenfrequency solver and you have your modes