Geting post title in textholder placeholder

Hi, I am using houze theme, and it has a option to change agent contact form to cf7, While adding a text area. I wanted to show current agents name in placeholder like what is displayed in theme form.

theme form is using this code to do that.

<div class="form-group">
                    <textarea name="message" id="message" rows="3"
                              class="form-control"><?php echo sprintf(esc_html__('Hi %s, I saw your profile on %s and wanted to see if you could help me', 'houzez'), $agent_name, get_option('blogname')); ?></textarea>
                </div>

How can i do that in contact form 7 form ?
I have added this as your massage filed.

<div class="form-group">
[textarea* your-massage class:form-control placeholder "Hi GET_$agent_name, I saw your profile on Krieta Homes and wanted to see if you could help me"]
</div>

but its not workinbg. anyone please help