BOSTON, Jan 3 (Reuters) - Activist investor Jana Partners is increasing pressure on customer service platform Zendesk (ZEN.N) to drop plans to buy Momentive Global (MNTV.O), the parent of SurveyMonkey, arguing the deal is too expensive and that shareholders oppose it. Jana said new documents show that the acquisition of Momentive appears to have been made only after Momentive"s advisors reached out to Zendesk at the end of August 2021. Jana wrote in a letter to the board, seen by Reuters, that it does not appear to be part of a long-considered strategy. The New York based firm, which is a top 10 shareholder in Zendesk, also criticized Zendesk"s plans to issue equity with a fair value of roughly $6 billion to Momentive shareholders to consummate the deal, the letter said. Zendesk agreed in late October to buy Momentive in an all-stock deal, which at the time was valued at nearly $4 billion. A representative for Zendesk was not immediately available for comment. Zendesk chose to issue the equity at an "artificially depressed price" before reporting strong earnings, the letter said. Jana cites Zendesk"s financial advisor, Goldman Sachs, who calculated Zendesk"s fair value at $176 a share, which compares to $103 per share in early trading on Monday. Jana also believes even if all goes well with a deal, the payoff would be slim, writing it would add a "meager 1% to Zendesk"s top line growth and about 10% to (its) future stock price" "Zendesk"s proposed acquisition of Momentive appears to be a reactive and impulsive decision," the letter said. Jana said it wants Zendesk to stop wasting time and money on a deal that it expects shareholders to reject and urged the board to "immediately terminate the transaction and pivot to focusing on far more promising avenues to resolve Zendesk’s growing discount." Jana wrote the letter after Zendesk filed its preliminary proxy in early December and roughly one month after the investment firm first pushed the board to scuttle the deal. Both Zendesk and Momentive investors would have to agree to the deal. The vote is expected to occur in late February or early March. Jana Partners said it has been in touch with more shareholders and wrote on Monday, "It should now be glaringly obvious to Zendesk"s board that shareholders do not support the proposed acquisition."