We present polarimetric observations of the Keyhole Nebula in the Carina Nebula Complex carried out using the Stratospheric Observatory for Infrared Astronomy. The Keyhole Nebula located to the west of $eta$ Carinae is believed to be disturbed by the stellar winds from the star. We observed the Keyhole Nebula at 89 $mu$m wavelength with the HAWC+ instrument. The observations cover the entire Keyhole Nebula spanning 8$$ by 5$$ with central position RA = 10:44:43 and Dec = -59:38:04. The typical uncertainty of polarization measurement is less than 0.5% in the region with intensity above 5,500 MJy sr$^{-1}$. The polarization has a mean of 2.4% with a standard deviation of 1.6% in the region above this intensity, similar to values in other high--mass star--forming regions. The magnetic field orientation in the bar--shaped structure is similar to the large--scale magnetic field orientation. On the other hand, the magnetic field direction in the loop is not aligned with the large--scale magnetic fields but has tight alignment with the loop itself. Analysis of the magnetic field angles and the gas turbulence suggests that the field strength is $sim$70 $mu$G in the loop. A simple comparison of the magnetic field tension to the ram pressure of $eta$ Carinaes stellar wind suggests that the magnetic fields in the Keyhole Nebula are not strong enough to maintain the current structure against the impact of the stellar wind, and that the role of the magnetic field in resisting stellar feedback in the Keyhole Nebula is limited.
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