Hi Enrique,
it seems that I misunderstood you before.
You're not sending a HTTP POST via B1i: You're receiving a HTTP Post with B1i!
Please check online Help for "HTTP Inbound".
To read out e.g. the first attribute from the payload Role=S, just add an XFORM transformation atom und use XSLT logic:
/vpf:Msg/vpf:Body/vpf:Payload[./@Role='S']/bfa:io/bfa:key[./@name='user']/@value
Best regards
Bastian