The inside cover of the WT and A magazines state: "To make a donation, please visit www.jw.org"
There is a link in the footer section of the website labeled: "Make a Donation to Our Worldwide Work"
This link goes to: https://apps.jw.org/E_DONATE
In addition the rank and file can donate to their own local congregation via https://khdonate.net
BoE Letter April 17, 2016 - Re: Arrangements for Electronic Donations to Local Congregations