Formal:
* Envoyer un message: This is the most common and formal way to say it.
* Transmettre un message: This emphasizes the act of passing the message along.
Informal:
* Faire passer un message: This literally means "to pass along a message."
* Dire un message: This means "to say a message" and is used when you are referring to verbally conveying a message.
For specific types of messages:
* Envoyer un SMS: To send a text message.
* Envoyer un courriel: To send an email.
Examples:
* Pourriez-vous envoyer un message à mon frère ? (Could you send a message to my brother?)
* J'ai un message à vous faire passer. (I have a message for you to pass along.)
* Je vais envoyer un SMS à Marie. (I'm going to send a text message to Marie.)
Choose the phrase that best fits your situation and level of formality.