Show the shape
Suppose "a title and two short sentences" still leaves the style unclear.
You can supply a completed example:
"Seed swap"
"Bring your spare seeds to the garden room on Sunday, 10:00 to 12:00. Bring one labelled packet; booking is required at [booking address to confirm]."
It is like handing someone a filled-in form. The example makes the layout and level of detail visible.
Putting a few worked examples into a prompt is called
Say which parts to follow: "Use this structure, but take all event facts from the repair café brief." Otherwise the model has to infer whether the day, location, and booking rule should carry over too.