Custom attribute in B2C Signup page

I have created custom attributes to my Azure B2C directory, and would now like to do a custom signup page but cannot figure out how to "map" the ui element to the custom attribute, please help.

From the example my guess is I need to use "extension_" suffixed witht the field name in the element id? The documentation is thin in this area

<div class="attrEntry">
    <div class="helpText"></div>
    <label>Loyalty number</label>
    <input id="extension_MemNum" class="textInput" type="text" placeholder="Loyalty number"><a href="javascript:void(0)" onclick="selfAssertedClient.showHelp('Membership number');" class="tiny">What is this?</a>
</div>