Can you write a personall message for your clients ? 

A few inspirational sentences about how excited you would be to work with them, how this work is your baby that you we been building for a while now an you cannot wait to start it and for you and your clients to grow together. Or it can be a bit more oriented towards the free 30min call, how you want to hear from them, whether or not you decide to work together afterwards 
we can also add your signature here. So it's sort of a personal touch , like a letter from you to them -with idea that they will want to reply to your 'letter' by asking for a discovery call =)