Optimus

- He told you one night after a romantic date

Ratchet

- He told you when you both were spending time together

Bumblebee

- He wrote you a cute note one day while you both were hanging out

Arcee

- She told you while you both were watching the sunset

Smokescreen

- He ordered a pizza and wrote it on the inside of the pizza box lid

Bulkhead

- He ( tried ) to write you a song and sing it to you

Wheeljack

- He rigged fireworks to spell it in the sky

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Klunklover1999 gave me the idea of 'how they tell you they love you'