Posting data to a cell using Google Sheets API

Question

I'm pretty new to this sort of stuff and I have spent an entire day trying to figure out how to post data to a cell using the Sheets API...

Sheets API - Change contents of a cell

The above documentation asks me to create an entry element containing the relevant data (replacing key, worksheetId, and cell with their appropriate values). I have set Content-Type to application/atom+xml. My code is as follows...

<entry xmlns="http://www.w3.org/2005/Atom" xmlns:gs="http://schemas.google.com/spreadsheets/2006">
  <id>https://spreadsheets.google.com/feeds/cells/MY_KEY/od6/public/full/R2C2</id>
  <link rel="edit" type="application/atom+xml" href="https://spreadsheets.google.com/feeds/cells/MY_KEY/od6/public/full/R2C2"/>
  <gs:cell row="2" col="2" inputValue="35"/>
</entry>

Using Postman, I am able to GET data from spreadsheet but when I try to PUT the data I get

"Google Drive -- Page Not Found".

Any ideas what I am doing wrong?

Thanks!