If you want someone else to submit a Declaration on your behalf, you should visit the Government Gateway and create the other person within the organisation as a new user or an assistant.
If a person has been enrolled on the Government Gateway and find their details incorrect they can:
The user or assistant can now change their details accordingly.
The person submitting the Declaration must log in as themselves and not someone else. If you pass your Online Government Account to another person, you will be responsible for any actions they carry out, as they will be recorded against you.