To better understand the trigger mechanism of a coronal mass ejection (CME), we present the evolution of a CME source region (active region NOAA 12790) and the formation of a hot channel before the occurrence of the first halo CME in solar cycle 25. Through analyzing the evolution of Solar Dynamics Observatory/Helioseismic and Magnetic Imager line-of-sight magnetograms, it is found that continuous magnetic cancellation occurs at the polarity inversion line (PIL) in this active region. With ongoing magnetic cancellation, several bidirectional jets and unidirectional jets occur along the large-scale arched magnetic loops. A hot channel forms during the first bidirectional jet. After the occurrence of the fourth bidirectional jet, the hot channel immediately erupts and produces a C-class flare, a cusp structure, and a halo CME. It is worth pointing out that the cusp structure only appears in the 131 angstrom and 94 angstrom observations (temperature about 10 MK). The obvious contraction of the newly formed loops is observed at the top of the cusp structure. The observations reveal a clear physics process: magnetic cancellation of a bipolar magnetic field at the PIL results in the occurrence of the bidirectional/unidirectional jets and the formation of the hot channel. The axial magnetic flux feeding for the hot channel through the continued magnetic cancellation leads to the hot channel eruption, which results in the formation of the hot cusp structure and the occurrence of the C-class flare and the halo CME.