On Form 1310, enter the trustee's name.  For non-spouse trustee, pick C for Part I and 3 for Part II.  Make sure the listed individual has the full understanding that the refund is issued to him/her as the trustee of the trust and will deposit the check to the trust.  Many financial accounts list the trustees' names on the accounts.

In order to process transactions involving a trust, many entities including the tax authorities find the need to examine the trust agreement, and prudently so.  Estate of a decedent is just a reasonable default.