https://donate.wikimedia.org/wiki/FAQ
This says:
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What is your refund policy?
If for any reason you wish to have your donation refunded, please contact us via email at donate@wikimedia.org[1] and include the following information:
Full name of donor
Date of donation — All refund requests must be made within 90 days of donation
Amount donated
Payment method used — Do not include credit card numbers in your email
Country of origin
Reason for the refund
All refunds will be processed as quickly as possible, but processing times may vary depending on the payment method. Please note: Some payment methods may not support refunds or require refunds to be made through the payment method (card) utilized, prompting additional information to process your refund.
[1] Due to the volume of inquiries we receive, we use Zendesk as a donor response platform. By emailing donate@wikimedia.org, you understand that your information will be processed by the Zendesk Group in accordance with Zendesk’s terms. Users from the following countries should consult Section A of Zendesk’s privacy policy for more information about Zendesk’s country-specific practices: Australia, Brazil, Canada, Japan, New Zealand, and Singapore.