Reciting Nishmat Kol Chai for Forty Days

Hello, I am not aware of any source that states there is a segulah to recite Nishmat Kol Chai for 40 days. However, it is known what the Ben Ish Chai wrote (Year 2, Parashat Toldot, section 3), that this praise of Nishmat Kol Chai is very precious and exalted, and it should be recited melodiously. It is also effective for any trouble a person is facing, and he should vow to recite this praise of Nishmat Kol Chai after being saved from the trouble, and it will help him, as written in the name of Rabbi Yehudah HaChasid z"l. This is also brought in Chazon Ovadia, Sukkot (p. 444, note 6), בשם שו"ת מכתם לדוד (Part Orach Chaim, siman 13), except that he wrote that he should say that he accepts it upon himself without a vow. With blessings, Hillel Meirs