...
1. Create a Service Request to Oracle in order to enable 1st party tracking for you
2. Create an Eloqua specific subdomain through your DNS settings for your main domain, e.g. tracking.example.com (first-party cookie domain)
3. Add one new line to Eloqua tracking script code on your website including the created Eloqua specific subdomain (see Oracle guidelines):
_elqQ.push(['elqUseFirstPartyCookie', '<tracking.example.com>']);
4. If you are using SSL, your tracking domain needs to be configured as a secure microsite in Oracle Eloqua and must have an SSL certificate associated to it (in practice either a "wildcard" certificate of your main domain or a new certificate purchased for the tracking domain).
5. You need to update the form action on all forms hosted outside Eloqua to use your tracking domain, for example: https://tracking.example.com/e/f2 instead of Eloqua based domain like: https://s123456789.t.eloqua.com/e/f2. Without this, individuals filling the form won't be getting proper 1st party cookie.
...
https://docs.oracle.com/cloud/latest/marketingcs_gs/OMCAA/Help/EloquaAsynchronousTrackingScripts/EloquaAsynchronousTrackingScripts.htm
...
...