I wanted to simulate Parallel Plates (2.1 Parallel Plates, 2.1.1 Introduction) with FDTD because I wanna know whether the simulation result will change due to differences in solution.
Then I'd like to set the frequency to 27MHz in setup and set constant potential to +50V and -50V in Boundary Condition.
Please let me know if you have a nice plan in setting the FDTD source.
Not entirely sure what you are referencing here, but you could sketch a line in your model environment (connected to the plates). Then in the simulation tab you can add this to the simulation you would like to run. Click on the multi-tree button in the menu bar and drag the line into the simulation. Specify that it is an "edge source". Then in the simulation go to the sources tab and fill in the frequency you want (27MHz).
As for the potential you want to set, you should just run the simulation and normalize the answer afterwards in the analysis tab.
Hopefully this is what you are looking for.