From UHC EDC Medicare Solutions Agent Guide (Version 2.3 Dated 2/5/2016) Page 99:

Agents are prohibited from completing the web enrollment on behalf of the consumer or at the consumer’s request. However, an agent may be on the telephone in order to assist the consumer with a web enrollment.
Agents must not be physically present with the consumer when a consumer is completing a web-based enrollment and must not engage in any screen sharing with the consumer through an internet connection (e.g., the consumer gives the agent control of the consumer’s computer to complete a web enrollment via WebEx) unless agreed to by the Vice President of Sales Oversight and the Compliance Officer.
Completing a web enrollment through the web-based enrollment tool on behalf of a consumer may be considered fraud.

According to the Medicare Marketing Guidelines Section 120.6:

However, if Plans/Part D Sponsors use licensed agents/brokers (employed or contracted) as customer service representatives, they cannot act as both a customer service representative and a sales/marketing agent/broker.

Translation:

Agents cannot use UnitedHealthcare consumer websites to enroll their prospects into UnitedHealthcare plans.

Medicare allows licensed agents to assist in enrollment as long as they are acting as a customer service representative