If you do not want to do the inpainting directly in the web interface, you'll have to upload a "alpha mask" of the area that you want to inpaint.
Here's an example:
Let's say this is your image. You open it in a editor that supports layers like Gimp. You add a transparent layer on top and draw the area that you want to inpaint over it in black. It should look like this.
Next, you replace the color layer with a white layer. It will look like this:
This is the image that you save and what you upload to the Automatic1111 web interface. Feel free to ask if you have any questions.