I guess this is what is valid json…can you try this please. Escaped "(double quotes) with \(backslash)
{
"query": "query {recentTransactions(last: 100, filter: {createdAt_gte: \"2022 - 12 - 12 T00: 00: 00\"}) {edges {node {createdAt transactionHeader {postedDate transactionNumber}text account {code}amount}}}}"
}
cheers