You need a payload which uses the primary key of the underlying table. In this case it’s the general journal line table. Something like this would do the trick:
{
"Journal_Template_Name": "GENERAL",
"Journal_Batch_Name": "DEFAULT",
"Posting_Date": "2019-07-31",
"Document_Type": "Payment”,
"External_Document_No": "",
"Account_Type": "Customer",
"Account_No": "110-49",
"Amount": 2000,
"Bal_Account_Type": "Bank Account”,
"Bal_Account_No": "962-05",
"Comment": "Web serivce posting",
"Shortcut_Dimension_1_Code": "PROPERTY",
"Shortcut_Dimension_2_Code": "21-01-001",
"ShortcutDimCode3": "80-50-025",
"ShortcutDimCode4": "",
"ShortcutDimCode5": "",
"ShortcutDimCode6": "",
"ShortcutDimCode7": "",
"ShortcutDimCode8": ""
}