How do i post data to url with fields?

You have to build the array as the json array needs to be

$args = array(
'headers' => array(
  'Content-Type' => 'application/json',
  'Authorization' => 'Basic '.$apiKey
)
);
$bdy = array(
    'fields'=> array(
      'project' => array(
        'key' =>'LD'
      ),
      'summary'=> 'CODING WORKS',
      'description' => 'Creating of an issue using project keys and issue type names using the REST API',
      'issuetype' => array(
        'name'=> 'Candidate'
      )
    )
);


$pload = array(
'method' => 'POST',
'timeout' => 30,
'redirection' => 5,
'httpversion' => '1.0',
'blocking' => true,
'headers' => $args,
'body' => json_encode($bdy)
);
$response = wp_remote_post ('https://mysite.atlassian.net/rest/api/2/issue',$pload);