Hello Rwjb, 

Are you able to log into that company, not through the accountant's account? 

Once you have logged into that account as a normal user, not as an accountant you will then be able to submit the vat via your credentials and not have to submit it through agents account. 

The reason why it says to do it through the asa is because you are logged into that account as an accountant, we need the system to think your not an account so you can then submit the vat via mtd with your credentials. 

If you have any quires more about this let us know.