The phrase "ambitious message" is correct and usable in written English.
It can be used to describe a message that is intended to inspire or motivate, often conveying a strong or lofty goal.
Example: "The CEO delivered an ambitious message about the company's future, aiming to rally the employees around a shared vision."
Alternatives: "aspirational message" or "bold message".
When he was a boy in the 1970s, his family would talk about his great-great-grandfather's ambitious message to people "belonging to me" 100 years into the future and, eventually, Thorpe received his own photocopy of the handwritten letter.
The two Voyager probes – both still going strong after 35 years – carry a far more ambitious message from Earth into interstellar space, including audio messages etched into golden records with instructions about how to play them.
NASA placed a more ambitious message aboard both Voyager probes, a special kind of time capsule intended to communicate the story of our world to extraterrestrials.
Now there's an ambitious, unifying message.
At my foundation, we are just about to launch an ambitious text-message service allowing the millions of Kenyan women who face gender-based violence to report it in confidence to police officers and paralegals the we are training up right now - some of whom are men who have signed themselves up to this cause.
So we must be ambitious for our message of Freedom, Fairness and Trust.
Entropy, by Maryam Khosrovani, is an ambitious work whose message of environmental degradation could hardly be more serious and whose craftsmanship is beyond impressive.
Mr. Cameron has made a swift, exciting special-effects epic" with "better developed characters (after all, the first Terminator was barely verbal), a livelier wit and a more ambitious, if nuttier, message".
In the longer term Facebook clearly has ambitious plans around messaging.
But world leaders seeking to show solidarity with Paris and to make progress against extremism can send an emphatic message with an ambitious agreement that helps prevent climate conditions that make it easier for groups like ISIS to take hold.
This has been put into practice most recently by the Global Goals initiative that aimed to reach seven billion people – the world's population - in seven days, spreading the message of 17 ambitious targets set by the United Nations - from ending extreme poverty through to securing gender equality.
