To get a list of available command just call redmine help.
The help command also features a dedicated help for each command.
Example
Note: Have you noticed the quotation marks encapsulating "In Progress"? Use these to escape values with whitespaces. Feel free to skip them for simple values, such as High.
Protips
Although the help command is pretty straightforward, here are some pro tips for you.
Open issue in default browser.
This tool is intended as an enhancement rather than a replacement of the Redmine web solution. So there are good reasons to use both in parallel. To make this as convenient as possible for you, there is the open command, which opens a specified issue in your default browser for you.
Which statuses, priorities and trackers are configured?
Redmine features the ability to configure certain properties of an issue. So there is no general assignment available.
This means, for example --priority=High might not be available on every Redmine instance. In order to filter, create or update you need to know the available assignments.
That's where the list command comes into the game.
How to obtain my API key?
In order to use this tool you are required to provide an API key for authentication.
Login into Redmine
Click "My account" (in the upper right corner)
On the right-hand-side panel you could find your API key within the API access key section
Copy this key.
Note: If you do not see this section, most probably the REST API access of your Redmine instance has been disabled. If you are the administrator of the server you could enable it in the server settings.