Tacx accounts need to be converted to Garmin Connect accounts to sync your Tacx activities with Strava.
Download the Tacx Training App.
If you're a new user, you can create an account that will automatically sync with Garmin Connect.
If you're an existing user, you will be prompted to connect your Garmin Connect account. Once you've done so, you can sign back into the Tacx Training App with your Garmin Connect credentials.
Set up your Garmin Connect account to automatically sync to Strava using the instructions outlined here.