Dear Sir/Madam,
Greetings of the day,
I hope you are doing well. I write this email in order to inform you that our __________ (college/ school/ university/ office/ company/other) is going to organize an event _____________ (name of the event) on __/__/____ (date). I am glad to inform that we have planned to organize the seminar on _______ (topic) which will help students/ employees in getting motivated and realize their goals.
In this regard, we would like to invite you as a guest speaker. It would be a moment of great honor for us to have you as a guest speaker. In case you find yourself interested in being a guest speaker, you may contact at the contact details mentioned, or you may respond to this email.
Thanking you,
__________ (mention name),
__________ (contact details)