Hi @Nancy12,
Yes, you can do this. I would list the fee you intend to charge for WIFI on your listing and make it very clear. When the guest books, find out if they want WIFI and if they do you can send them a request using the Resolution Center. You would choose the option "Request payment for extra services". Check out this How To guide below:
https://community.airbnb.com/t5/General-Hosting/How-To-Guide-on-Using-the-Resolution-Center/m-p/8580...
I would like to add that I do agree with @Margaret0 that most hosts include it in the price of their listing and for many guests, WIFI is an expectation. I do also understand that you may live where WIFI is extremely expensive and there may be a need to charge for it.
Happy hosting @Nancy12!
Dave
David
Superhost Ambassador ~ Host Club Community Leader ~ Community Expert ~ Experienced Co-Host