Here are the instructions from Miva...
"If you are using any external system which requires a Miva username or
password to connect to Miva you’ll need designate that user an API user which
will disable browser verification and forced 2-factor authentication.
Important: Its extremely important you only do these steps for users which are
used to connect to Miva via an external system (Synchro, Shipstation, Stone
Edge, Shipworks, Dynamic Order Export, etc) and not being used to login to Miva
by a real person. Doing so will greatly reduce the security of your ecommerce
store.
Steps
1. Login into to Miva
2. Ensure your store is updated to Miva 9.10
3. Navigate to Users and edit the user you wish to designate an API user
4. In the Description field for that user enter the text “api” and hit save
You can repeat this process for any other API users you may have."
-Kevin