How to implement document picker in ios?

I need to implement document picker in ios, im using visual studio 2017 and netstandard code strategy.
I'm using plugins and not works, always returns error "this functionality is not implemented ...." and
others dont do anything.
I've had too time in this and I have not found a response.
Im have the AppId, and the provisioning profile with the capabilities with Cloudkit, im have the configurations and the Entitlements.plist, Im have one interface to try to implement this, but in this part is on the i'm are in trouble, im never programming in objective c. Im have the archives in icloud in the iphone, i need to can select the archive and validate the extension.
Thanks in advance.