It seems there were/are a couple of workarounds for setting third party cookies in Safari.
One was to send a POST request in a hidden iframe using javascript. This was supposedly what Google used to bypass Safari's blocking of third party cookies[1].
Another is (was?) to redirect to the third party domain, and then back again[2]. This would supposedly work since the restriction on third party cookies doesn't apply to already visited domains.
Google have recently rolled out a 'global tag' to replace the 'floodlight' tracking code they usually use, this was last week.
[1] https://support.google.com/dcm/partner/answer/7570440?hl=en