You use the setsockopt system call. This is an example in C:

    int sfd = socket(domain, socktype, 0);

    int optval = 1;
    setsockopt(sfd, SOL_SOCKET, SO_REUSEPORT, &optval, sizeof(optval));

    bind(sfd, (struct sockaddr *) &addr, addrlen);

In Go, you can use syscall.SetsockoptInt. Most languages have a way of setting this option. You have to create the socket yourself and pass it into your HTTP server in most cases, but it depends on the library.

Edit: oh sorry, you meant when systemd is opening the port for you. It looks like you can set ReusePort=yes in your configuration? https://www.freedesktop.org/software/systemd/man/systemd.soc...