Link to autofill an input box in another page

I'm trying to create a link that when clicked on will redirect me to another html page with a contact form, and in that form there is a subject input box. Depending on which link is clicked, I want the subject box to be populated with the corresponding link title.

Tried searching for awhile now and have no idea how to get it to work. Tried playing around with the key value pairs method mentioned in some other places but definitely doing it wrong, and not sure how.

Below is a snippet of my code. The h3 "Title of the product here" is what I want to populate the subject box with (in my actual website I will have multiple links populating the subject lines with different words).

Main Page

<div class="squareIndexContentBox1" style="background-color: #fff">
    <a href="contact us.html?subject=testing">
        <h3>Title of the product here</h3>
        <p>
            Lorem ipsum dolor sit amet, consectetur adipiscing elit.
        </p>
        <div class="learn-more">
            <span id="learn-more">Contact us</span>
            <span class="fa fa-hand-o-left"></span>
        </div>
    </a>
</div>

Page where I want the input box to be filled

<div class="contact-form">
    <h2> Contact Us </h2>

    <form method="POST" action="contact-form-handler.php">
        <input type="text" name="name" placeholder="Name" required>
        <br>

        <input type="email" name="email" placeholder="Email" required>
        <br>

        <input id="subject" type="text" name="subject" placeholder="Subject" required>
        <br>

        <textarea type="text" name="message" placeholder="Message"></textarea>
        <br>

        <div class="g-recaptcha" data-sitekey=""></div>
        <br>

        <input type="submit" name="submit" class="submit-btn" value="Send Message">
    </form>
</div>

Thanks in advance!