HOW TO DEFINE COMMUNICATION OBJECTIVE?
You can gather your project team and brainstorm ideas. Try to answer questions: What do you want to achieve with each specific message? Which topics would attract young people outside urban area to participate in youth mobility?
For example, we are aware that young people outside urban area are interested in cross-cultural communication, developing social capital among young people and empowering ability to actively participate in society. So, in one of your messages, you can highlight international network that could be made through mobility.

If you want to be more specific you can add some goal numbers you wish to reach. For example, highlight international network to attract 5 new participants to your youth mobility program. However, have in mind that goals need to be realistic, do not overestimate your possibilities.
Remember: do not confuse communication objectives with those of the project itself.
Here are few examples of some general communication objectives you can use in your strategy:
Promote interpersonal relationship which could be reach through mobility
Motivate young people outside urban area to take part in student mobility program
Engage young people outside urban area to actively participate in social media post about specific mobility
Endorse once in a lifetime experience which young people outside urban area could gain through mobility