Here's an example of how you can parse JSONP (JSON with padding) string in Go:

Go: 
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package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"strings"
)

func main() {
	// JSONP string
	jsonp := []byte(`callbackFunction({"message": "Hello, World!"})`)

	// Get the position of the first '(' and the last ')'
	start := bytes.IndexByte(jsonp, '(')
	end := bytes.LastIndexByte(jsonp, ')')

	// Extract the JSON string from the JSONP string
	jsonString := jsonp[start+1 : end]

	// Unmarshal the JSON string into a Go map
	var data map[string]interface{}
	json.Unmarshal(jsonString, &data)

	// Print the message
	fmt.Println(data["message"])
}

In this example, we first extract the JSON string from the JSONP string by finding the positions of the first '(' and the last ')'. We then use the json.Unmarshal function to unmarshal the JSON string into a Go map. Finally, we print the "message" field of the map.