The desktop app can import cookies from another browser so you can reuse its signed-in sessions in the preview browser.
Open Settings → Integrations → Browser profiles → Add profile, then choose a browser under Import from. Close the source browser before importing, and allow an operating-system keyring unlock prompt if one appears.
This is a one-time copy. Later login changes stay separate between the two browsers, and some sites may still require you to sign in again.
On macOS, Safari imports need Full Disk Access. Choose Allow, drag T3 Code into the System Settings permission list, and turn access on. Continue becomes available when access is detected. macOS may require you to quit and reopen T3 Code before the grant applies; reopen the import wizard afterward. You can revoke Full Disk Access once the import is done.
On Windows, import supports Firefox and Helium profiles that use standard profile encryption. Other Chromium-based browsers use app-bound encryption and cannot be imported. Partitioned cookies are skipped on all platforms.