Yes, this can be achieved using the FusionAuth API. Here's how you can implement it:

Set the default password for each user:
Use the POST /api/user/change-password endpoint to set the user's password to the desired format, e.g., company1234. Refer to the "Change a User's Password using an email address or username" section of the FusionAuth documentation. This allows you to update a user's password in a single API call without requiring the Forgot Password flow. Require the user to change their password:
Use the PATCH /api/user/{userId} endpoint to set the passwordChangeRequired field to true. This ensures that the user will be prompted to change their password upon their next login.

API Documentation for reference:

Change a User’s Password Update a User

Optional Customization:
To guide users on the login page to use the default password format, you can customize the messaging through FusionAuth Themes:

Navigate to Simple Theme > Messages > Default. Add a custom message, such as password=Password is company+username, to inform users of the default password structure.