Morning and welcome,
Sorry about the delay in getting to your question. Using the get_prior function will return the default priors that brms sets. RT models aren’t in my wheelhouse but there is a good discussion here on RT model and setting priors Advice on distribution for modelling reaction times? (and setting priors)
As for setting priors I usually think about what previous data or knowledge we have. Like if you know that as the answer length goes up, reaction time also increase you would want to set a prior that reflects this. Typically I comment the references for each prior directly into the code.
Does that make sense?