The t is silent, as in Harlow.
Notes from Wikiquote
Correcting Jean Harlow's pronunciation of Margot. It is sometimes said to have been the actress Margot Grahame who delivered this rebuke.
Quote text and notes from Wikiquote: Margot Asquith (page history), available under CC BY-SA 4.0. Source quality and labels are our own summary of what Wikiquote records.