Here is something you can try.
Dim objDBWheres As New List(Of DbWhere)
Dim objDBWhere As New DbWhere("RegisterID",DbOperator.IsEqualTo,"E000204")
objDBWheres.Add(objDBWhere)
Dim objScnWhere As New DbWhere("WFScenarioName",DbOperator.IsEqualTo,"BudgetV1")
objDBWheres.Add(objScnWhere)
Dim strFirstName As String = BRApi.Database.LookupRowFieldValue(si, "App", "XFW_PLP_Register", objDBWheres, "FirstName","")
So this will create a WHERE similar to RegisterID='E000204' AND WFScenarioName='BudgetV1' (code does RegisterID=@RegisterID AND WFScenarioName=@WFScenarioName)
You can use AppendWithORVersusAND to change the default of AND to an OR. I don't think anyone will be using an alternate parameter name. So you can create the object then use a set for ORVersusAnd rather than mentioning the parenthesis and alternate name.