code: public function setHeader($keyName, $value)
      {
          $keyName = $this->normalizeHeaderName($keyName);
          if (!$keyName) {
              return $this;
          }
  
          if (!isset($this->configs['headers'])) {
              $this->configs['headers'] = [];
          }
  
          $this->configs['headers'][$keyName] = [$value];
          if (!$this->inProcessingLoop) {
              return $this
                  ->buildConfigClient()
                  ->withRequest($this->request->withHeader($keyName, $value));
          }
  
          return $this
              ->withRequest($this->request->withHeader($keyName, $value));
      }
docs: Set Header
  
  @param string $keyName Header Name key, The key will be convert into
  First Character after `-`(dash) into uppercase
  And space will be replace as `dash`
  @param string $value
  @return static