FxPortal Bridge is similar to POS Bridge with a feature of using ERC standardized tokens without mapping.
For more info about FxPortal, please read - FxPortal Bridge doc
FxPortal bridge functionality is available in seperate repository as a plugin for maticjs. The package provides different API's for interacting with bridge.
Here is repository link - https://github.com/fx-portal/maticjs-fxportal