package main

import (
	"fmt"
	"strings"
)

func main() {
	str := "a,b,c"
	mySlice := strings.Split(str, ",")
	mySlice = append(mySlice, "1")              // append
	mySlice = append([]string{"2"}, mySlice...) // prepend
	fmt.Println(mySlice)
	fmt.Println(strings.Join(mySlice, ","))
}

mySlice = append([]string{"2"}, mySlice...)