############### Code
<table width="350" border="0" align="center" cellpadding="0" cellspacing="0">
<tr>
<td><form action="" method="post" name="form1">
<table width="350" border="0" cellspacing="5" cellpadding="0">
<tr>
<td width="105">Full Name</td>
<td width="16">:</td>
<td width="222"><input name="fullname" type="text" id="fullname">
* </td>
</tr>
<tr>
<td>E-mail</td>
<td>:</td>
<td><input name="email" type="text" id="email">
* </td>
</tr>
<tr>
<td>Phone number </td>
<td>:</td>
<td><input name="phone_number" type="text" id="phone_number">
* </td>
</tr>
<tr>
<td> </td>
<td> </td>
<td><input type="submit" name="Submit" value="Submit">
</td>
</tr>
</table>
</form></td>
</tr>
</table>
Click on table and you will see form tag appeared then click to select it 
At your panel bar open your tag inspector panel (press F9 or window > Tag Inapector)
Select tab "Behaviors"
Select onSubmit
Click on a plus sign then select "Validate Form"
In this step in Name fields box select what field you don't want to be blank then tick "Required" and choose what you accept. 
Press submit button without insert any value in form fields you will see this alert box. 
