Nil Rate Band Carryforward
Visit the Taxes screen, on the People panel, right, select the name of the person who is to receive the transferred nil rate band. Alternatively, if that person already has an entry in the Taxes screen, select their entry from the Ledger.
Then expand IHT Exemption / Nil Rate Band Carry Forward and set the Percent of IHT Nil Rate Band carried forward to 100%, and the Percent of Residence Nil Rate Band carried forward to 100%, if applicable, (or the relevant percentages).
Click Add or Update.