🐛 Send credentials on body when exchanging the code for the access token (#1169)

packages/nodes-base/credentials/MailchimpOAuth2Api.credentials.ts

@@ -49,7 +49,7 @@ export class MailchimpOAuth2Api implements ICredentialType {
 			displayName: 'Authentication',
 			name: 'authentication',
 			type: 'hidden' as NodePropertyTypes,
-			default: 'header',
+			default: 'body',
 		},
 	];
 }