Set the form ID:
<form id="primo_postcodes_form">
Place inside <form>:
<label>Search:</label>
<input id="postcode" name="postcode" type="text">
<button id="btn-postcode-search" type="button">Find Address</button>
<div id="primo_postcodes_results"></div>
<label>Address Line 1:</label>
<input id="line_1" type="text">
<label>Address Line 2:</label>
<input id="line_2" type="text">
<label>Town/City:</label>
<input id="town_or_city" type="text">
<label>County:</label>
<input id="county" type="text">
Note: Form ID, input field attributes are all customisable, using primoPostcodesSettings variables.