While a dependency for all systems, it generally is not already installed on Windows.
By default, Git will add the necessary directory to your PATH (the "recommended" option), and as long as you do not choose the "Use Git from Git Bash Only" option when installing, Portmod will be able to find your Git installation automatically.
Finally, you will also need to install Sandboxie, the Sandbox tool Portmod interfaces with on Windows (see Sandbox). Note that Portmod automatically configures Sandboxie; you do not need to, and indeed should not, modify the sandbox configurations generated by Portmod.
The sandboxie program files directory needs to be added to your PATH environment variable so that it can be detected by patool.
It is worth noting that, while Portmod in general does not require administrator privileges, you will need administrator privileges to create symlinks on certain versions of Windows (newer versions of Windows 10 should support creating symlinks without privileges). Currently the only symlink portmod requires is the profile symlink, so when you select a profile using omwselect profile set, you will need to ensure that omwselect is run with administrator privileges.