The classic office card gets stuck on somebody's desk. A digital card removes the obstacles, but you still need a good plan.
1. Start early
Give people a week or more. Most people write in the last two days anyway.
2. Share one link
Post the link in the team chat, not in an e-mail that is easy to miss.

3. Tell people what you want
"A memory, a thank you and a wish" gets better results than "please sign".
4. Ask for a photo
Photos make the card; ask for one specifically.
5. Remind once
One friendly reminder two days before the deadline works well.

6. Keep the date secret
Schedule the delivery so the recipient is surprised. You can still add messages until the day itself.
7. Add your own message first
An example from the organiser makes it easier for others to start.
