No .. just no!
In all seriousness - never share your login credentials for any online service with anyone and that includes MYOB.
MYOB has provided developers with safe and secure ways to allow you to grant permission to your data, it's called OAUTH. Officially reviewed and authorised add-ons (all of which you can find on myob.com/addons) make use of this technology to ensure they can add value to your business life while protecting your data.
If you are asked for your details, say NO. Then report them directly to us on email@example.com as we will want to have a conversation with them about security.