// You need to add two includes to the top of the file
#include <boost/property_tree/json_parser.hpp>
#include <boost/property_tree/ptree.hpp>

// after checking for application/json
std::string json_str = beast::buffers_to_string(req_.body().data());    
std::stringstream           ss(json_str);
boost::property_tree::ptree json_tree;
boost::property_tree::read_json(ss, json_tree);
std::string timestamp = json_tree.get<std::string>("timestamp");
std::cout << "timestamp: " << timestamp << std::endl;
for (const auto & waypoint : json_tree.get_child("waypoints")) {
    float lat = waypoint.second.get<float>("latitude");
    float lon = waypoint.second.get<float>("longitude");
    std::cout << "waypoint latlon: " << lat << " " << lon << std::endl;
}