New way

Step 1: In your league's discord server, find the My Madden bot in the members list or from one of the messages it sends when replying to a command

Step 2: send a dm to it the My Madden bot in this format using the email address you registered on MyMadden with. 

connect email@domain.com

Old way

Step 1: Go Enable developer mode by following the screenshots

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Step 2: Copy your Discord id by right clicking or long pressing on your name or avatar

Step 3: Add it to your MyMadden profile