Hi guys, i done this a few weeks ago but i cant remember the syntax to populate a labels text heres is my HTML

Can someone help me with the syntax its the labels that have required written in them its something lilke 'span[id*=#MainContent_lblUserName' i cant remember the exact syntax

<div class="accountInfo">
       <p style="margin-left:16%">
                <label for="MainContent_txtUserName" id="MainContent_UserNameLabel">Username:</label>
                <input name="ctl00$MainContent$txtUserName" type="text" id="MainContent_txtUserName" class="textEntry" />&nbsp<span id="MainContent_lblUserName">required</span>
            </p>
            <p style="margin-left: 16%">
                <label for="MainContent_txtPassword" id="MainContent_PasswordLabel" style="padding-right:4.5px">Password:</label>
                <input name="ctl00$MainContent$txtPassword" type="password" id="MainContent_txtPassword" class="passwordEntry" />&nbsp<span id="MainContent_lblPassword">required</span>
            </p>
        <p class="submitButton">
            <input type="submit" name="ctl00$MainContent$LoginButton" value="Log In" id="MainContent_LoginButton" />
        </p>
    </div>