!!! Overview
[{$pagename}] ([form_post]) is the [Response_mode] [parameter] which indicates is an [OAuth 2.0] [Response_mode] where [Authorization Response] [parameters] are encoded as [HTML] form values that are auto-submitted in the [User-agent], and thus are transmitted via the [HTTP POST] method to the [Client], with the result parameters being encoded in the body using the [application]/x-www-form-urlencoded format.

The action attribute of the form [MUST] be the [OAuth Client]'s [redirect_uri]. The method of the form attribute [MUST] be [POST|HTTP POST]. 

Because the [Authorization Response] is intended to be used only once, the [Authorization Server] [MUST] instruct the [User-agent] (and any intermediaries) not to store or reuse the content of the [Authorization Response].

Any technique supported by the [user-agent] [MAY] be used to cause the submission of the form, and any form content necessary to support this [MAY] be included, such as submit controls and client-side scripting commands. However, the [OAuth Client] [MUST] be able to process the message without regard for the mechanism by which the form submission was initiated.

!! [OAuth 2.0 Security Considerations]
As described in [OAuth 2.0 Multiple Response Type Encoding Practices] [OAuth.Responses], there are security implications to encoding response values in the [URI Query] string and in the [URI Fragment Identifiers] value. Some of these concerns can be addressed by using the [{$pagename}]. In particular, it is safe to return [Authorization Response] parameters whose default [Response_modes] are the query encoding or the fragment encoding using the [form_post] [Response_mode].

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [OAuth 2.0 Form Post Response Mode|https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html|target='_blank'] - based on information obtained 2017-06-26-