Code design for handle funcs in go web app

I'm learning go and ran into some design issues while developing web app. The app has main route "/" where user can submit a simple form. With those form values I am calling external API and unmarshaling response into some struct. Now from here I want to make another call based on retrieved values to another external API and I'm not sure what's the proper way of doing this. Here is a snippet for better understandment:

func main() {
    http.HandleFunc("/", mainHandler)
    log.Fatal(http.ListenAndServe(":8080", nil))
}

func mainHandler(w http.ResponseWriter, r *http.Request) {
    //renders form template
    //makes post and retrieves data from api
    //here with retrieved data I want to make another call to different API,
    // but mainHandler would get too big and complex. I'm not sure how should I pass this data to
    // another handler or redirect to another handler with this data.
}