Right now this feature is not available in B2C sign up page using JS. You can add it in support request if you think ,this is required feature in forum.Please make sure to implement custom policy if the request can be fulfilled using policies and it 's a recommend way too.
Here is the guideline for using javascript in B2C sign up page
Guidelines for using JavaScript
Follow these guidelines when you customize the interface of your application using JavaScript:
• Don't bind a click event on HTML elements.
• Don’t take a dependency on Azure AD B2C code or comments.
• Don't change the order or hierarchy of Azure AD B2C HTML elements. Use an Azure AD B2C policy to control the order of the UI elements.
• You can call any RESTful service with these considerations:
o You may need to set your RESTful service CORS to allow client-side HTTP calls.
o Make sure your RESTful service is secure and uses only the HTTPS protocol.
o Don't use JavaScript directly to call Azure AD B2C endpoints.
• You can embed your JavaScript or you can link to external JavaScript files. When using an external JavaScript file, make sure to use the absolute URL and not a relative URL.
• JavaScript frameworks:
o Azure AD B2C uses a specific version of jQuery. Don’t include another version of jQuery. Using more than one version on the same page causes issues.
o Using RequireJS isn't supported.
o Most JavaScript frameworks are not supported by Azure AD B2C.
• Azure AD B2C settings can be read by calling window.SETTINGS, window.CONTENT objects, such as the current UI language. Don’t change the value of these objects.
• To customize the Azure AD B2C error message, use localization in a policy.
• If anything can be achieved by using a policy, generally it's the recommended way.