I failed to request an access token.

Following [doc to authentification using JWT](https://help.salesforce.com/articleView?id=remoteaccess_oauth_jwt_flow.htm&type=5), I already have my connected app, my JWT Key so I just need to make a POST requestion to : `https://test.salesforce.com/services/oauth2/token`

Params are on the body : 

```
grant_type : urn:ietf:params:oauth:grant-type:jwt-bearer
assertion : the content of my jwt.key (private key)

```

I'm having error 400 :

```
{
"error": "invalid_grant",
"error_description": "invalid assertion"
}

```

Also added : `Content-Type : application/x-www-form-urlencoded`

What should I correct/add ?

* `assertion : the content of my jwt.key (private key)` ? shouldn't be the private key of my certificat ?
* add `authorization => basic auth` and ask my administrator to send me username/password of my connected app ?