I'm with a few agencies, all non-exclusive and wonder how my child would be able to continue taking my sales if I'm gone. No intention of that happening any rime soon!!!
I've included specific details of my stock sites in my will, which should be enough for agencies.
However, this has been asked often enough before, and it may be (?) that different agencies have different requirements.
I know in the past there has been official word from Getty that they need this to be specified in a will; though normally unless otherwise specified, that should be part of the residue of an estate. But different countries have different rules/laws. Why not ask your lawyer?