In HTML, you can use the 'required' attribute to specify that a form field should be filled in before submitting the form. However, this is not directly applicable to HTML itself.

<input type='text' name='username' required>

However, if you want to make a field mandatory in your HTML form, you would typically use JavaScript or CSS for validation and styling. You can also use server-side programming languages like Python or PHP to check the field's value before submitting the form.

For example, if you're using JavaScript, you can add an event listener to the input field that checks its value and displays an error message if it's empty. Then, when the form is submitted, you can check again for a non-empty value before submitting.