optin form firstname and lastname required ?

default opt-in form only required email
but now, I want first name and last name required too
how to edit it

default code
<form action="http://domain.com/index.php/lists/yh427j9t0recb/subscribe" method="post" accept-charset="utf-8" target="_blank">

<div class="form-group">
<label>Email <span class="required">*</span></label>
<input type="text" class="form-control" name="EMAIL" placeholder="" value="" required />
</div>

<div class="form-group">
<label>First name</label>
<input type="text" class="form-control" name="FNAME" placeholder="" value=""/>
</div>

<div class="form-group">
<label>Last name</label>
<input type="text" class="form-control" name="LNAME" placeholder="" value=""/>
</div>

<div class="clearfix"><!-- --></div>
<div class="actions">
<button type="submit" class="btn btn-primary btn-flat">Subscribe</button>
</div>
<div class="clearfix"><!-- --></div>

</form>
 
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