How to add attribute declaration in XSD for element with sequence of child elements?

An attribute declaration can be added within xs:complexType after the xs:sequence:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
           elementFormDefault="qualified">
  <xs:element name="Address">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="StreetAddress" minOccurs="0" type="xs:string"/>
        <xs:element name="OtherDestination" minOccurs="0" type="xs:string"/>
        <xs:element name="City" minOccurs="0" type="xs:string"/>
      </xs:sequence>

      <!------------------------------------------>
      <!-- This is where to declare attributes: -->
      <xs:attribute name="id" type="xs:string"/>
      <!------------------------------------------>

    </xs:complexType>
  </xs:element>
</xs:schema>

The above XSD will validate your XML successfully.