where target_domain is the domain against which you are invoking this call, for example, intuit.quickbase.com.
So you can make a report where the filter is that checkbox being true and hide the report form all role so no one is likely to mess with it. i would also put in the description of the report to not delete or alter the report.
var text URL = urlroot() & "db/" & [_DBID_TABLE name goes here] & "?act=API_PurgeRecord&qid=99";
You can put that into a URL formula field. If you want to have a dashboard button, make the URL formula on a field somewhere in the app and expose the code by by suppressing the link text. Then copy and paste the code into a URL dashboard button element.
I would test this first on a Copy of your app, as when you purge records they are "gone baby gone".
There is also another way to alter the URL to not depend on a report qid, but rather just use query parameters. That would actually be a bit safer as that way you would not have a risk of someone removing the filers on your report and then the purge might purge everything.