There is no solution. The desktop Lightroom doesn't support this. You have to export a jpeg image and then send that. If you have an iPhone or iPad and creative cloud, you can sync the image you want to send through iMessage to Lightroom mobile and there you can very easily share the image through SMS or iMessage.
Thank you Jao.
I have LR Mobile so I guess I'll just use that. I didn't realise you could just sync a single image to LR Mobile, thought it was all or nothing and I don't have enough storage on my phone for that. I'll look into it.
You can sync one image, a collection, etc. alA good trick for your purpose is to make a collection (call it "images to text" for example) that is synced. If you want to text an image just drag it into this collection. It will get synced to your phone and you can just text from there. Also don't worry about storage too much. The mobile LR intelligently manages this and pulls down from the cloud what it needs.