The high number of seniors that feels excluded of society highlights the necessity to promote active ageing. This intention can be supported through online platforms that encourage participation in social activities. The goal of the present study was to identify design principles of online support platforms for seniors through focus groups and to ideate the platform through co-creation sessions. The results show that a social platform for seniors must, among other, help to foster contact between users, and must provide services and meaningful activities. A first mock-up of the platform's design has been created based on the co-creation sessions and will be iteratively evaluated and enhanced in future studies in four countries across Europe. Our findings are in line with those of other studies demonstrating that seniors attach importance to trustworthiness and need to maintain social ties.