Ask HN: How to submit cover letter on recruiting sites

This question is triggered by looking at some of the jobs listed in the "Who's Hiring" post, and seeing that many which use sites like:

https://boards.greenhouse.io

to do the actual submittion only seem to allow one attachment for a CV/Resume, with no ability to write anything or attach anything else for a cover letter.

In the past (last time I looked for jobs was 3 years ago), many of the job boards allowed you to submit additional cover letter attachments (or at least had a large textbox for a general description), so what would the recommendation be for sites like: https://boards.greenhouse.io ?

Modify the CV .pdf to customise it / include the cover letter pre-amble, and submit that? Or do nothing and just submit the CV (that seems less than ideal, but maybe it gets ingested and stripped away anyway)?