How to append byte array to slice of bytes in Go

I have a simple bit of code that i am trying to understand, but am struggling to work out how to get it to work properly.

The general idea is i want to pass some data, and convert it into a byte array. Then i want to apply the length of the byte array at the first index of my byte slice, then add the byte array to end of the slice.

This is what it tried:

    var slice []byte
    myString := "Hello there"

    stringAsByteArray := []byte(myString) //convert my string to byte array


    slice[0] = byte(len(stringAsByteArray)) //length of string as byte array


    append(slice, stringAsByteArray) 

So the idea is the first byte of slice contains the number of len(b) then following on from that, the actual string message as a series of bytes.

But i get:

cannot use stringAsByteArray (type []byte) as type byte in append
append(slice, stringAsByteArray) evaluated but not used

Answer

For example,

package main

import "fmt"

func main() {
    myString := "Hello there"

    slice := make([]byte, 1, 1+len(myString))
    slice[0] = byte(len(myString))
    slice = append(slice, myString...)
    fmt.Println(slice[0], string(slice[1:]))
}

Output:

11 Hello there