func main() {
    //...

    // create custom HTTP client with custom Transport
    client := &http.Client{
        Transport: &http.Transport{},
    }

    // create HTTP request
    req, err := http.NewRequest("GET", "https://httpbin.io/user-agent", nil)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    // set User-Agent header
    req.Header.Set("User-Agent", randomUA)

}