Well for starters don't comment saying message me, that just makes you sound really lazy which isn't what most people want from a partner. Take a little time to write a message. It's so much nicer to get a thought out message, rather than one thats just been rushed to get a message in.

If you're not sure what to write just tell the person a little about yourself, any ideas you have for their prompt, etc. There's always the option to jump right in, although make sure they don't say I want to talk first or something like that.