Apex Restを作っていてJson の値をポストした時のエラーです。

{
  "subject" : "Wingo Ducks",
  "Description" : "707-555-1234",
  "TaskSubtype" : "Task",
  "OwnerName" : "Wingo Ducks",
  "Status" : "Completed",
  "Date ActivityDate" : "2024-12-18",
  "supplier_code" : "10005"
  "customer_code" : null,
}

400エラーですね

[{"message":"Unexpected parameter encountered during deserialization: subject at [line:3, column:16]","errorCode":"JSON_PARSER_ERROR"}]

以下の例をみると... icって定義がありますね。
https://developer.salesforce.com/docs/atlas.ja-jp.apexcode.meta/apexcode/apex_rest_methods.htm

もしかして、calss名を追加する？

{
Body053 :{
  "subject" : "Wingo Ducks",
  "Description" : "707-555-1234",
  "TaskSubtype" : "Task",
  "OwnerName" : "Wingo Ducks",
  "Status" : "Completed",
  "Date ActivityDate" : "2024-12-18",
  "supplier_code" : "10005"
  "customer_code" : null,
}
}

エラーは変わらずです。
もしかしてb53aの変数と一緒にしないといけないのかな？

    @HttpPost
    global static string doPost(Body053 b53a){
        system.debug(Logginglevel.INFO,'==========> '+ b53a);

{
b53a :{
  "subject" : "Wingo Ducks",
  "Description" : "707-555-1234",
  "TaskSubtype" : "Task",
  "OwnerName" : "Wingo Ducks",
  "Status" : "Completed",
  "Date ActivityDate" : "2024-12-18",
  "supplier_code" : "10005"
  "customer_code" : null,
}
}

エラーが変わりました。あっ、日付型のところの項目名がおかしい

[{"message":"Unknown field: CreateTaskRESTAPI.Body053.Date ActivityDate at [line:8, column:26]","errorCode":"JSON_PARSER_ERROR"}]

"Date ActivityDate"　--> "ActivityDate" に変更

[{"message":"Unexpected character ('"' (code 34)): was expecting comma to separate OBJECT entries at [line:10, column:4]","errorCode":"JSON_PARSER_ERROR"}]

"10005"の後にカンマが無いな。

無事に解決です。思ったより悩みました。

List型に対応させる

    @HttpPost
    global static string doPost(List<Body053> b53List){
        system.debug(Logginglevel.INFO,'==========> '+ b53List);

Postするデータは以下のフォーマット

{
"b53List" :[{
  "subject" : "Wingo Ducks",
  "Description" : "707-555-1234",
  "TaskSubtype" : "Task",
  "OwnerName" : "Wingo Ducks",
  "Status" : "Completed",
  "ActivityDate" : "2024-12-18",
  "supplier_code" : "10005",
  "customer_code" : null
},
{
  "subject" : "Wingo Ducks2",
  "Description" : "707-555-1234",
  "TaskSubtype" : "Task",
  "OwnerName" : "Wingo Ducks",
  "Status" : "Completed",
  "ActivityDate" : "2024-12-18",
  "supplier_code" : "10005",
  "customer_code" : null
}
]}