Here’s a template letter of how to do so under GDPR. You must request your data or the deletion of you data using the correct legal framework (quoting the correct legislation) and these templates make this easy. Plus they cover more types of data than just your posts and comments.

https://www.datarequests.org/sample-letters/

You could consider making a data request first and after they respond, make a deletion request.

Once I've got my data under gdpr I'll start with those mass comment edit services (shreddit, powerdeletesuite etc) and do several overwrites. Then I'll request (demand) them ro nuke all my data. Obliterate it.