Assuming you have alreay onboarded your agency partners (see How To Onboard Your Agency Partners), you can use a template like this to brief them on a specific communications project.

It is helpful at the briefing stage to ask for examples of how the idea could be used across multiple channels to make sure it has the flexibility the brand needs.

Large agencies usually take a brief like this from the client (as well as the onboarding brief) and use what they've learned to brief and inspire their creatives.

You can find a useful discussion about this topic in this LinkedIn thread.