You mean Section 645.  You might need to file Form 8855 (but not with a return).  

To make an election under IRC Sec. 645, go to Screen 69 Election to Treat a Qualified Revocable Trust as Part of an Estate and complete the required info. Print out the completed Form 8855 and mail it to the address shown in the form instructions.

It is important to file Form 8855 no later than the due date for filing the Form 1041 for the first tax year of the trust. The election form will be considered timely filed if it is filed by the extended due date for filing the Form 1041, provided the trust has been granted an extension for the first tax year of the trust.