At this time, the Core extension supports “Native Segwit” addresses for BTC withdrawals and these start with “bc1q”. For more information on the types of BTC addresses please visit this link.
For a more in-depth read on how the Avalanche Bridge™ works, please read the following articles.
For any additional questions, please visit our knowledge base or contact a support team member via the chat button at support.avax.network.