Lol might not be something you want to encourage
BUT
I would do something that would trigger Bonnie to bark, in her case it was ring the doorbell and when she barked I said 'speak' and treated I did this loadssss of times and then tried it without the doorbell, just saying speak and when she did I treated........... I must admit she picked it up very quickly!!! and after she had mastered the speak command I added a hand signal.