Welcome to the Community, @anne40.
I can share some information about using Making Tax Digital (MTD) in QuickBooks Online.
Yes, you'll need to authorize QuickBooks again to interact with HMRC. After doing it, you can use your existing HMRC login credentials to grant authority.
First, I suggest going to the Taxes section to enable the feature. For the detailed steps and screenshots, you can visit the Authorise QuickBooks to interact with HMRC - MTD for small businesses article.
Additionally, I recommend the following resource, which contains information about MTD:
Please know that I'm just a post away if you have any other questions. Have a great day ahead.
