Jan 11 (Reuters) - Zendesk Inc (ZEN.N) investor Janus Henderson Investors said on Tuesday it did not see a strategic advantage to the software firm"s proposed acquisition of Momentive Global Inc (MNTV.O), joining activist investor Jana Partners in opposing the deal. Zendesk agreed in late October to buy Momentive, the parent of online survey portal SurveyMonkey, in an all-stock deal, which at the time was valued at nearly $4 billion. read more "We are gravely concerned about the strategic, tactical, and financial rationale of this proposed deal," Janus Henderson Investors said in a proxy statement. (https://bit.ly/3fgHuQU) "We think the company should instead renew its focus on executing on the (Zendesk) standalone business." The activist investor did not disclose the size of its stake in Zendesk, but said it was a large, multi-year shareholder in the company. Refinitiv data showed Janus Henderson held a 4.94% stake in Zendesk, which had a market capitalization of $12.21 billion as of stock"s last close. Zendesk, a customer service software vendor, has seen a surge in business during the pandemic as its enterprise customers accelerate digital transformation. In November, Jana Partners urged Zendesk to abandon its deal for Momentive. read more Shares of Zendesk were up marginally in trading after the bell, while those of Momentive were flat.