code: public RecipientDsnFields withExtensionFields(ExtensionField... extensionFields) {
          if (this.extensionFields == null) {
              setExtensionFields(new com.amazonaws.internal.SdkInternalList<ExtensionField>(extensionFields.length));
          }
          for (ExtensionField ele : extensionFields) {
              this.extensionFields.add(ele);
          }
          return this;
      }
docs: <p>
  Additional X-headers to include in the DSN.
  </p>
  <p>
  <b>NOTE:</b> This method appends the values to the existing list (if any). Use
  {@link #setExtensionFields(java.util.Collection)} or {@link #withExtensionFields(java.util.Collection)} if you
  want to override the existing values.
  </p>
  
  @param extensionFields
  Additional X-headers to include in the DSN.
  @return Returns a reference to this object so that method calls can be chained together.