I think you need to create unbound action using codeunit to create a custom here is an example from Microsoft documentation https://learn.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-creating-and-interacting-with-odatav4-unbound-action

    codeunit 50100 "MiscOperations"
    {
    procedure Ping(input: Integer): Integer
    begin
        exit(-input);
    end;
 
    procedure Delay(delayMilliseconds: Integer)
    begin
        Sleep(delayMilliseconds);
    end;
 
    procedure GetLengthOfStringWithConfirmation(inputJson: Text): Integer
    var
        c: JsonToken;
        input: JsonObject;
    begin
        input.ReadFrom(inputJson);
        if input.Get('confirm', c) and c.AsValue().AsBoolean() = true and input.Get('str', c) then
            exit(StrLen(c.AsValue().AsText()))
        else
            exit(-1);
    end;
}