In an item's text field, if you enter !p3, Todoist will assign priority 3 to the item. Once you save the task, !p3 will be deleted from the text, and the item will be listed as priority 3.
Any priority level can be assigned with this shortcut, such as !!1 for priority 1, !p2 for priority 2, or !p4 for priority 4. For example: