In a post can be shared only public trainings passed or started by the current API user. To retrieve the list of these trainings, you can call getTrainingsForSharing query. The query accepts the same arguments as getCatalogData and can be paginated, sorted and filtered in the same way. It also re turns exactly the same structure as getCatalogData. 
  query getTrainingsForSharing {
    getTrainingsForSharing(first: 2) {
      edges {
        node {
          id
          training {
            title
            image {
              alt
              title
              url
            }
          }
        }
      }
      pageInfo {
        hasNextPage
        startCursor
        endCursor
      }
    }
  }