Just like OP did. Use the Appeal guided entry and explain your case.
https://eu.battle.net/support/en/article/2639
If you're not using 1 blizzard account, explain what your other accounts are and what you were doing.
Do not use multiboxing as your only excuse, that's not what you were banned for.
Do not lie, they know more about your accounts than you probably do.
If you were up to no good, cut your loses and move on. Stop w/e you were doing. Multiboxing has enough issues than to be dragged into the mud with RMTers etc.
Good luck.
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