What did the Federalists promise to add to the Constitution in order to win support for its ratification?

Because the wanted guaranteed protection for certain basic liberties, such as freedom of speech an trial by jury.

They wanted guaranteed protection for certain basic liberties, such as freedom of speech and trial by jury. A Bill of Rights was added in 1791. In part to gain the support of the Anti-Federalists, the Federalists promised to add a bill of rights if the Anti-Federalists would vote for the Constitution.