I think the simplest solution is to modify the link helper macro in the theme. This macro looks like this by default:
[#macro link url extraParameters=""]
<a href="${url}?tenantId=${(tenantId)!''}&client_id=${(client_id?url)!''}&nonce=${(nonce?url)!''}&redirect_uri=${(redirect_uri?url)!''}&response_mode=${(response_mode?url)!''}&response_type=${(response_type?url)!''}&scope=${(scope?url)!''}&state=${(state?url)!''}&timezone=${(timezone?url)!''}&metaData.device.name=${(metaData.device.name?url)!''}&metaData.device.type=${(metaData.device.type?url)!''}${extraParameters!''}&code_challenge=${(code_challenge?url)!''}&code_challenge_method=${(code_challenge_method?url)!''}&user_code=${(user_code?url)!''}">
[#nested/]
</a>
[/#macro]
What we want to do is modify the redirect_uri in certain cases. In this case, we know that the url will have the value /password/forgot so we can put an if statement in there:
[#macro link url extraParameters=""]
[#if url == "/password/forgot"]
[#assign redirect_uri="https://example.com/pageb"]
[/#if]
...
You'll also need to:
create the page that lives at url B (https://example.com/pageb in this example). It won't be hosted by FusionAuth.
add url B to the list of Authorized Redirect URLs in your OAuth configuration.
That should work. There are some other alternatives, however. If you generate the "Forgot password" page without a client_id, you won't end up logged in after a password reset. Instead you'll end up at the "OAuth password complete" page, which you can design in the theme editor.
More about themes.
