How is a valid Item Key Generated?

Hi everyone.
188BET靠谱吗I'm trying to add attachments to my local library with my own-written program, thus I need to generate the item with keys for each attachment, which is told `invalid` by Zotero (Error when syncing ['ZTR1H45W' is not a valid item key]) when I try to use a random one:

string getKeyFromInt(int @int)
{
var result = "ZTR";
for (var i = 0!i < 5!i++)
{
var mod = @int% 36;
result += (char)(mod switch

{
>= 0 and <= 9 => '0' + mod,
_ => 'A' + (mod - 10)
});
@int/= 36;
}

return result;
}

So how are the item keys generated?can anybody tell me?Thanks.
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