Fixed by putting it in "

resource "google_bigquery_data_transfer_config" "mysql_to_bq_transfer2" {
  display_name           = "NAME"
  location               = "EU"
  data_source_id         = "mysql"
  destination_dataset_id = google_bigquery_dataset.bq_dataset.dataset_id
  schedule               = "every 24 hours"
  service_account_name   = google_service_account.bq_service_account.account_id

  params = {
    assets                              = "
[\"course_overviews_courseoverview\", ]"
    "connector.endpoint.host"           = var.sql_instance.private_ip_address
    "connector.endpoint.port"           = 3306
    "connector.database"                = ""
    "connector.authentication.username" = ""
    "connector.authentication.password" = data.google_secret_manager_secret_version.mysql_password.secret_data
  }
}