James wrote

For the electrostatics module, four boundary conditions can be found: electric potential, Ground/antisymmetry, surface charge and insulation/symmetry. If I want to use the free electrical potential boundary condition (that is, electrical potential can be changed with time and space during the simulations due to contributions from the test domain), I cannot find the options for this implementation.

I am not quite sure how you define your "free electrical potential boundary condition" as I couldn't find a specific definition of this (ideally please attach the equation for the boundary condition you are looking for), also please clarify what you mean by "test domain" (is it the computational domain or something else)? In general Neumann or flux type boundary conditions (here the surface charge and insulation/symmetry conditions) does not prescribe or fix a value for the dependent variable (electric potential) but instead in essence prescribes a gradient for the solution, allowing the dependent variable take which ever value fulfills this gradient.

James wrote

However, if I choose interior boundaries, there will be continuity option

Yes, the default BC for interior boundaries is "continuity", which means that the gradient of the solution from the two sides/domain will be continuous (have no jump).

James wrote

Does it mean I have to add a domain outside the outer boundary so that I can know how it changes with time and space?

Unfortunately, I'm not sure what you mean or are asking about here either. If you could attach a detailed problem description with sketch illustrating what you want to do that would help I think.