How to add a book...

I can't seem to remember how to add a book that I need added to my reading challenge.  It doesn't exist in the catalog (I don't think) but I know it can be done...just can't remember how.  Anyone that can point me in the right direction please?

 

Just in case I'm not finding it but it is there-

https://www.amazon.co.uk/gp/product/B00GA83MRY/ref=x_gr_w_bb_sin?ie=UTF8&tag=x_gr_w_bb_sin_uk-21&linkCode=as2&camp=1634&creative=6738