Trello: Attach a link

Probably the most important button in your card is the ATTACHMENT button.
"Trello is 𝐚 𝐜𝐨𝐥𝐥𝐚𝐛𝐨𝐫𝐚𝐭𝐢𝐨𝐧 𝐭𝐨𝐨𝐥 that 𝐨𝐫𝐠𝐚𝐧𝐢𝐳𝐞𝐬 𝐲𝐨𝐮𝐫 𝐩𝐫𝐨𝐣𝐞𝐜𝐭𝐬 into boards. In one glance, Trello tells you what's being worked on, who's working on what, and where something is in a process.
Imagine a whiteboard, filled with lists of sticky notes, with each note as a task for you and your team. Now imagine that each of those sticky notes has photos, attachments from other data sources like BitBucket or Salesforce, documents, and a place to comment and collaborate with your teammates. Now imagine that you can take that whiteboard anywhere you go on your smartphone, and can access it from any computer through the web. That's Trello!”

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[brenncaballeromusic]

How can i link two cards automatically when i add an attachment to one?