you can do it with jquery
at first, create the button, you can style it with css
<div id="message-toggle">Send Message</div>
then hide the form and give an id for jquery
<div id="message-container" style="display: none;" class="widget-content seller-message">
then you need jquery to open the formular
<script>
$(document).ready(function(){
$(document).on("click", "#message-toggle", function(){
$("#message-toggle").slideToggle("slow", function(){
$("#message-container").slideToggle("slow");
});
});
});
</script>
and the whole code should look like this
<div class="widget">
<h3><i class="fa fa-envelope"></i> <?php _e('Send message', 'glider'); ?></h3>
<div id="message-toggle">Send Message</div>
<div id="message-container" style="display: none;" class="widget-content seller-message">
<form action="<?php echo osc_base_url(true); ?>" method="post" name="contact_form" id="contact_form" <?php if (osc_item_attachment()) { echo 'enctype="multipart/form-data"'; }; ?> >
<?php osc_prepare_user_info(); ?>
<input type="hidden" name="action" value="contact_post" />
<input type="hidden" name="page" value="item" />
<input type="hidden" name="id" value="<?php echo osc_esc_html(osc_item_id()); ?>" />
<div class="form-group">
<input type="text" name="yourName" id="yourName" placeholder="<?php echo osc_esc_html(__('Your name', 'glider')); ?>">
</div>
<div class="form-group">
<input type="text" name="yourEmail" id="yourEmail" placeholder="<?php echo osc_esc_html(__('Your e-mail address', 'glider')); ?>">
</div>
<div class="form-group">
<input type="text" name="phoneNumber" id="phoneNumber" placeholder="<?php echo osc_esc_html(__('Phone number', 'glider')); ?> (<?php echo osc_esc_html(__('optional', 'glider')); ?>)">
</div>
<div class="form-group">
<textarea rows="4" name="message" id="message" placeholder="<?php echo osc_esc_html(__('Message', 'glider')); ?>"></textarea>
</div>
<?php if(osc_item_attachment()) { ?>
<div class="form-group">
<label for="attachment"><?php _e('Attachment', 'glider'); ?>:</label>
<?php ContactForm::your_attachment(); ?>
</div>
<?php } ?>
<div class="form-group">
<?php if (function_exists('anr_captcha_form_field')) { ?>
<?php anr_captcha_form_field(); ?>
<?php } else { ?>
<?php echo responsive_recaptcha(); ?>
<?php osc_show_recaptcha(); ?>
<?php } ?>
</div>
<button type="submit" class="btn btn-block btn-default"><?php _e("Send", 'glider'); ?></button>
</form>
<?php SellerValidation(); ?>
</div>
</div>
<script>
$(document).ready(function(){
$(document).on("click", "#message-toggle", function(){
$("#message-toggle").slideToggle("slow", function(){
$("#message-container").slideToggle("slow");
});
});
});
</script>