To make this possible you'll need the two following strategies turned on:
Making a Fast Add Button
To create this button head over to the Fast Add Button settings. There choose to add a new item and then make it look like this.
By doing this once you type the name of the task it will be automatically scheduled for today.
Adding the button to the Skinny Sidebar
Now go to Skinny Sidebar settings, click on "Add new item" -> "Fast Add Button" and choose the one you just made. Also, feel free to choose an icon that you like, as that's the icon that will show up in the Skinny Sidebar.
Here's a short GIF of how this works: