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 under-represented groups to participate in youth mobility?
For example, we are aware that underrepresented groups that have not usually participated in youth mobility may not understand the benefits of participating or may not feel connection with another participants. Becoming familiar with what matters to groups you seek to engage will help you adapt your messaging to these audiences.

If you want to be more specific you can add some goal numbers you wish to reach. For example, encouraging members of underrepresented groups to participate in youth mobility will attract 5 new participants. 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:
Encourage all members of underrepresented groups to include in youth mobility society
Engage them to actively participate in social media post about specific mobility
Inform underrepresented group members about possibilities
Highlight interpersonal value gained through mobility