Invalid signature problem

I follow step by step this tutorial OAuth-1.0-Server to get the request token but I can't.

Assuming that my oauth_consumer_key=123 and my consumer_secret=456. My request would be the next:

POST /oauth/initiate HTTP/1.1
Host: api.openbankproject.com
Authorization: OAuth
oauth_callback=oob,
oauth_consumer_key="123",
oauth_signature_method="HMAC-SHA256",
oauth_timestamp="1522987628",
oauth_nonce="ErFKKs"

So, I encode the parameters in this way (Following the instructions):

• First, the http method:
  POST&

• Then, the base string URI encoded:
  POST&https%3A%2F%2Fapi.openbankproject.com&

• Finally, I get the base string after normalizing the request parameters according to Section 3.4.1.3.2:
  POST&https%3A%2F%2Fapi.openbankproject.com&oauth_callback%3Doob%26oauth_consumer_key%3D123%26oauth_nonce%3DErFKKs%26oauth_signature_method%3DHMAC-SHA256%26oauth_timestamp%3D1522987628

• The secret is created by the concatenation of consumer_secret and oauth_consumer_key in this way:
  456&123 (consumer_secret&oauth_consumer_key)

• So, if I sign the base string with the secret 456&123 using HMAC-SHA256 I get:
  9a055becbf91403ec0ecc73f574862a7c9e77fcd27650ec5a530fbbfabe1f948

• And after encoding to Base64 I get:
  OWEwNTViZWNiZjkxNDAzZWMwZWNjNzNmNTc0ODYyYTdjOWU3N2ZjZDI3NjUwZWM1YTUzMGZiYmZhYmUxZjk0OA==

And it doesn't work !!! I cannot get my request token, my signature is wrong.
Am I doing something wrong ? Please, I need help

Start with the OAuth-1.0-Server tutorial and the POST /oauth/initiate request described in the issue, then compare the documented signature construction with the server's expected OAuth parameters. No repository file or test is named; done means identifying the cause of the rejected signature and documenting a confirmed correction or reproduction.