Solr 8 upgrade and stream.body - solr

Question

I'm upgrading Solr from 6.x to 8.x. In the past, we used to build our request thusly in our PHP script:
$aPostData = array(
'stream.body' => '{"add": {"doc":{...stuff here...}}',
'commit' => 'true',
'collection' => 'mycollection',
'expandMacros' => 'false'
);
$oBody = new \http\Message\Body();
$oBody->addForm($aPostData);
sending it to our Solr server at /solr/mycollection/update/json. That worked just fine in 6.x but now that I've upgraded to 8.x, I'm receiving the following response from Solr
{
"responseHeader":{
"status":400,
"QTime":1
},
"error":{
"metadata":[
"error-class","org.apache.solr.common.SolrException",
"root-error-class","org.apache.solr.common.SolrException"],
"msg":"missing content stream",
"code":400
}
}
Digging around I ran across the following
https://issues.apache.org/jira/browse/SOLR-10748
and
Solr error - Stream Body is disabled
I tried following the suggestions of both answers. For the first one, I now see a file called "configoverlay.json" in my ./conf directory and it has those settings. For the second answer, I set it up so my requestParsers node had those attributes. However, neither worked. I've searched around but at this point I'm at my wits end. How can I make it so that I can continue using "stream.body"? If I shouldn't be using "stream.body" is there some other request var that I can/should use when sending my data? I couldn't find anything in the documentation. Perhaps I was looking in the wrong place?
Any help would be greatly appreciated.
thnx,
Christoph