Where can I enter my app specific password on appcenter.ms testflight integration?

You can follow the instructions here: https://docs.microsoft.com/en-us/appcenter/distribution/stores/apple#adding-two-factor-authentication

Specifically:

Adding Two-factor authentication

If your Apple account has two-factor authentication enabled, App Store Connect requires an app-specific password as security. You can add an App-specific password to your account by navigating to Developer accounts in your Account settings.

• Hover over an item in the Accounts list.
• Click the three vertical dots on the right side of the list
• Select Update app-specific password.
• Generate an app-specific password using the Apple ID portal.
  • The name is for you to remember which service or app is using the app-specific password.
• Copy the generated app-specific password and paste it into the dialogue.
• Save by clicking Update.