how to filter content inside message body,display differently in different page?

I have two page sharing the same contact form,
one with product enquiry list+ contact,
one with only contact
i append the product enquiry list using javascript in the hidden empty product enquiry field.

so in the message body i have extra field inside my contact form
…
..
Product Enquiry
[your-enquiry]
..
..
.

Is there other way to do without putting a constant value, and hack the sendmail by filter it out using str_replace("Product Enquiry <br/>No Enquiry ","",string)
before sending through wp_mail();

for example, if no product the the javascript will append “No Enquiry” to this hidden field, so in the contact page

….
Product Enquiry
No Enquiry
.
can be filtered out

https://www.ads-software.com/extend/plugins/contact-form-7/