Pre-population comment fields in donation pages through URL

Hi Everyone. I'm doing a charity ride for an organization that is using donation pages developed by Blackbaud and I was wondering if anyone had experience with adding a preamble text to a comment field in the donation field. For example, on my donation page:

https://secure.pwatoronto.org/site/Donation2?idb=2140389594&df_id=1480&FR_ID=1090&mfc_pref=T&PROXY_ID=1060281&PROXY_TYPE=20&1480.donation=form1

there is a “Leave a Comment” textarea near the bottom. As part of the charity fundraiser, I arranged raffles with a few local stores; people enter the draw by scanning a QR code and make a donation. I want the comment field to have the store's name pre-populated. As I draw names I will know which store the draw is for.

I was looking at Blackbaud's online help and found their help on hidden inputs:

https://webfiles-sc1.blackbaud.com/files/support/helpfiles/luminate-online/help/luminateonline.html?_ga=2.141715696.370563817.1711289765-1645215217.1711289765#../Subsystems/donation_management/Content/Concepts/Admin_Online_Giving_Management_DonationForms_HiddenInputs.html

And thought something like passing (say) “&set.Custom.Message=TBR” (where TBR is the store name) would do the trick, but I can't find the correct URL setter for this.

Any suggestions? The HTML for the “Leave a Comment” on the donation page is:

<div class="form-content">
<label for="tr_message_to_participantname">Leave a Comment:</label>
<textarea cols="35" rows="4" name="tr_message_to_participantname" id="tr_message_to_participantname" class=""></textarea>
<input type="hidden" name="tr_message_to_participantsubmit" id="tr_message_to_participantsubmit" value="true">
</div>