How use BIG-IP to capture HTTP request and response

To capture request POST you can try this.

 when HTTP_REQUEST {
    if { [HTTP::method] equals "POST" } {
        catch { HTTP::collect [HTTP::header Content-Length] }
    }

}
when HTTP_REQUEST_DATA {
    log local0. "[HTTP::payload]"
}

But I guess the option to troubleshoot for APM SSO, could be to use HTTPWatch or Fiddler.