Black should play the first move in the lower-right half of the upper-right corner. If the position remains symmetrical, White should play the second move in the upper-left half of the entire board. This is etiquette whether in person or online. The rationale is that most people are right handed and will naturally extend their right hand over these parts of the board, and you want to grant your opponent maximum comfort for playing the first moves as a sign of goodwill.
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