you won't increase the demand by doing that! you just show the demand that already exists. if those people who are willing to spend bitcoin on sites like Amazon can show their interest so that Amazon can know how many are out there who want this new feature.

you can not lie about it either. because if you show fake interest and they finally add bitcoin and it were not as big as it was claimed then it would be removed and will create a big shame. for example if you make fake demands for addition of ETH you will just embarrass yourself as nobody uses that for payments.