How to get POSTed JSON in Flask?

I'm trying to build a simple API using Flask, in which I now want to read some POSTed JSON. I do the POST with the Postman Chrome extension, and the JSON I POST is simply {"text":"hello world"}. I try to read the JSON using the following method:

@app.route('/api/add_message/', methods=['GET', 'POST'])
def add_message(uuid):
    content = request.json
    print(content)
    return uuid

On the browser it correctly returns the UUID I put in the GET, but on the console, it just prints out None (where I expect it to print out the {"text":"hello world"}. Does anybody know how I can get the posted JSON from within the Flask method?