VBA

A function you can use in a worksheet or a macro: the date n business days after tradeDate in the given calendars.

Public Function SettlementDate(tradeDate As Date, n As Long, calendars As String) As Variant
    Dim url As String
    url = "https://api.financecalendarapi.com/v1/business-days/add" & _
          "?date=" & Format(tradeDate, "yyyy-mm-dd") & "&n=" & n & _
          "&calendars=" & calendars & "&format=text"

    Dim http As Object
    Set http = CreateObject("MSXML2.XMLHTTP")
    http.Open "GET", url, False
    http.setRequestHeader "Authorization", "Bearer YOUR_KEY"
    http.send

    If http.Status = 200 Then
        SettlementDate = CDate(http.responseText)
    Else
        SettlementDate = CVErr(xlErrValue) ' the response body says why
    End If
End Function

In a cell: =SettlementDate(A2, 2, "USD,XNYS").

Errors come back as JSON problem details; read http.responseText to see the detail. Every request counts against your plan: for a long column, load a holiday list once (see Excel) and use WORKDAY.