Paste this into your chrome console

try_it_for_yourself.js

// Paste this into chrome console
var buffer_1 = new ArrayBuffer(50000);
var buffer_2 = new ArrayBuffer(1000000);
var needle = new Uint8Array(buffer_1);
var haystack = new Uint8Array(buffer_2);
needle.fill(111)
haystack.fill(222)
needle_length = needle.length;
haystack_length = haystack.length;
needle_length_string = needle_length.toString();
for (i = needle_length_string.length; i < 10; i++) {
    needle_length_string = "0" + needle_length_string;
}
if (needle_length < 1 || needle_length > 9999999999 || needle_length > haystack_length) {
    alert("The needle's length is not correct, must be between 1 and 9999999999");
} else {
    var base_array = needle_length_string.split('');
    for (i = 0; i < base_array.length; i++) base_array[i] = +base_array[i] | 0;
    console.log(base_array);
}
var buffer_to_go = new ArrayBuffer(10 + needle_length + haystack_length);
var array_to_go = new Uint8Array(buffer_to_go);
array_to_go.set(base_array);
array_to_go.set(needle, 10);
array_to_go.set(haystack, 10 + needle_length);

var url = "https://dev.rpc.ssvm.secondstate.io:8081/api/run/251/search_bytes_single_input";

var xhr = new XMLHttpRequest();

xhr.onreadystatechange = function() {
    if (xhr.readyState === 4) {
        console.log(xhr.response);
        $('#result').html(xhr.response);
    }
}

xhr.open('POST', url, true);
xhr.setRequestHeader("Content-Type", "application/octet-stream");
xhr.send(buffer_to_go);