Jan 13 (Reuters) - Software firm Zendesk Inc (ZEN.N) on Thursday reiterated the benefits of its proposed acquisition of Momentive Global Inc (MNTV.O) and asked shareholders to vote in its favor, right after two activist investors opposed the deal. In a letter to shareholders, Zendesk restated mostly only synergies that were disclosed earlier about the all-stock deal to buy Momentive, the parent of online survey portal SurveyMonkey. "The Zendesk board"s determination to acquire Momentive is the result of a thoughtful and deliberative review process spanning multiple alternatives within adjacent and complementary markets," the company said. However, activist investor Janus Henderson Investors said on Tuesday it did not see a strategic advantage to the software firm"s proposed acquisition and advised the company to renew its focus on executing on its standalone business. read more Janus Henderson holds a 4.94% stake in Zendesk, according to Refinitiv data. Meanwhile, another activist investor, Jana Partners, increased pressure on Zendesk to drop its acquisition plans, arguing that the deal was too expensive. At the time the deal was announced, in October, it was valued at nearly $4 billion. read more Zendesk said it has "increased conviction in the upside and the expected synergies" of the deal which is expected to close in the first half of this year.