I never get any response from Okta support team for this topic. Okta documentation is not straightforward and you need to struggle a lot to collect information required specially for Xamarin(iOS,Android and UWP).
You can try following sample if that works for you. It is a basic OidcClient library which works for all 3 platforms in Xamarin.Forms . For me, after implementing it was not giving me valid Okta token(i.e signature was not being verified from Okta) because for that I need some customization with Okta Authorization end point and this library doesn’t provide any customization.(i.e I need to pass some extra parameter for Authorize endpoint as per my server configured for Okta)
From this article you can find this link
for sample code.
But I end up having fully working Okta with manually calling all required Okta endpoint and I followed this documentation.
Authorization Code Flow with PKCE
Basically you need to call endpoints /authorize , /token ,/userinfo , /logout .
Hope this helps and good luck!!