REST API USE

Here is a connector BR we used to import from workday. You can use this as a starting point/reference and build your BR and use this connector BR in a DataSource. Original code provided by OneStream (courtesy Joakim Kulan)

Imports System
Imports System.Data
Imports System.Data.Common
Imports System.IO
Imports System.Collections.Generic
Imports System.Globalization
Imports System.Linq
Imports Microsoft.VisualBasic
Imports System.Windows.Forms
Imports OneStream.Shared.Common
Imports OneStream.Shared.Wcf
Imports OneStream.Shared.Engine
Imports OneStream.Shared.Database
Imports OneStream.Stage.Engine
Imports OneStream.Stage.Database
Imports OneStream.Finance.Engine
Imports OneStream.Finance.Database
Imports System.Net
Imports System.Web.Script.Serialization
Imports newtonsoft.json
Imports newtonsoft.json.linq
Imports System.Xml

Namespace OneStream.BusinessRule.Connector.FTE_Workday
	Public Class MainClass
		Public Function Main(ByVal si As SessionInfo, ByVal globals As BRGlobals, ByVal api As Transformer, ByVal args As ConnectorArgs) As Object
			Try
				'Get the Field name list or load the data 
				Select Case args.ActionType
					Case Is = ConnectorActionTypes.GetFieldList
						'Return Field Name List if using Field List Method to get field list in GetFieldList
						'-----------------------------------------------------------
						Dim fieldList As List(Of String) = GetFieldList(si, globals, api)
						Return fieldList
					Case Is = ConnectorActionTypes.GetData
						Dim sourceDataREST As DataTable = GetSourceDataREST(si, globals, api)
						api.Parser.ProcessDataTable(si, sourceDataREST, False, api.ProcessInfo)
						Return Nothing
				End Select
				Return Nothing
			Catch ex As Exception
				Throw ErrorHandler.LogWrite(si, New XFException(si, ex))
			End Try
		End Function
		
		'Create the field list items to get field list (Recommended)
		'-----------------------------------------------------------
		Private Function GetFieldList(ByVal si As SessionInfo, ByVal globals As BRGlobals, ByVal api As Transformer) As List(Of String)
			Try
				Dim fields As New List(Of String)
								      
				fields.add("Field1")
				fields.add("Field2")
				..
				fields.add("FieldN")

				Return fields
			Catch ex As Exception
				Throw ErrorHandler.LogWrite(si, New XFException(si, ex))
			End Try
		End Function

		'Create the data load Statement
		Private Function GetSourceDataREST(ByVal si As SessionInfo, ByVal globals As BRGlobals, ByVal api As Transformer) As DataTable
			Try
'				Dim wdUser As String = ""
'				Dim wdPass As String = ""
'				Dim wdCreds As String = encodebase64(wdUser & ":" & wdPass)
'				Use already encoded credentials (eg.,In Postman) key
				Dim wdCreds As String = "<encodedkey>"
				'Call the REST API		
				Dim wc As New WebClient
				wc.Encoding = Encoding.UTF8
				wc.Headers("cache-control") = "no-cache"
				wc.Headers("ContentType") = "application/json"
				wc.Headers("Accept") = "application/json"
				wc.Headers("Authorization") = "Basic " & wdCreds
												
				Dim jsonURL As String = "<webapiurl>?Format=json&<paramname>=" & <paramvalue>
				Dim json As String = wc.DownloadString(jsonurl)

				'Convert result in API to table format
'				Dim pulldata As DataTable = jsonConvert.DeserializeObject(Of DataTable)(json)
				Dim dataSet As DataSet = jsonConvert.DeserializeObject(Of DataSet)(json)
				Dim pullData As DataTable = dataSet.Tables(0)
				
				'Close Web connection
				wc.dispose
				'Return the dataset
				Return pullData
			Catch ex As Exception
			Throw ErrorHandler.LogWrite(si, New XFException(si, ex))
			End Try
		End Function
	End Class
End Namespace

Are you trying to import data into OneStream using an api call? or export data out of OneStream?