How to use fill_in with find in Capybara (if possible)

If you have a reference to the element itself you’d use set instead of fill_in:

find(:css, "input[id$='donation_pledge_hundreds']").set("10")

However for your specific example, fill_in should be able to find the element as you know it’s ID:

fill_in 'donation_pledge_hundreds', with: "10"