How can I attach a header to a datagram in c++ for networking?

I have a struct ip ipHeader;

that I want to attach in front of the actual data, which is string data;

so that I can send it in one packet via the send function, which takes in a char* buffer. How would I go about trying to concatenate these? I need to attach this ipHeader because it's a custom header that's different from the UDP header that's automatically attached when sending messages.