The input ___ attribute used to specify that an input field must be filled out before submitting the form.

The attribute you're referring to is the "required" attribute. This attribute is used in HTML forms to ensure that a certain input field is filled out by the user before they can submit the form. Here's how you can use it:

1. Identify the input field that you want to be filled out. This is done by using the <input> tag in HTML. For example, if you want to make an email field required, you would start with <input type="email">.

2. Add the "required" attribute to this input tag. You don't need to assign any value to it. Just including "required" within the tag is enough. So, your input tag now becomes <input type="email" required>.

3. Now, when the user tries to submit the form without filling out this email field, the browser will display a message asking them to fill out this field.

Remember, the "required" attribute works with the following input types: text, search, url, tel, email, password, date pickers, number, checkbox, radio, and file.