You do not request a step-up in basis, you report it as the basis for the sale on an income tax return.
In order to for the property to receive a stepped up basis, the property would need to have been included (or includible) in the decedent's estate for estate tax purposes (even if no estate tax was otherwise due). If the scenario is such that the property was simply a revocable living trust (e.g., a grantor trust), then there would generally be a step-up in basis upon the death of the grantor.
On the other hand, there would be no step-up in basis if, for example, the grantor had gifted the property into an irrevocable trust, inter vivos. The point here is you need to know how, when, and under what circumstances the property was placed in the trust.