insert

Inserts the string representation of the specified byte value into this string builder at the specified index and returns this instance.

@IgnorableReturnValueexpect fun StringBuilder.insert(index: Int, value: Byte): StringBuilder(source)

fun main() {
    val sb = StringBuilder("Hello")
    sb.insert(5, 65.toByte())   // inserts the string representation of the byte value
    println(sb)                  // prints: Hello65
}

Source