Google analytics 4 - Cross domain measurement

Hi,

I have a Squarespace website with a Booqable checkout which is embedded using JavaScript. I want Google Analytics 4 to maintain the same cookie for a customer when they enter on Squarespace, then checkout on Booqable. This link says that cross-domain measurement is the way to go - https://support.google.com/analytics/answer/10071811?hl=en.

Standard Google analytics works for the Squarespace site, but cross-domain tracking does not - when we go from Squarespace > Booqable, there is no _gl or _ga (cookie) values in the website URL as suggested in the article linked above. However:

• When we go from Booqable > Squarespace we do get the _gl & _ga values.
• If we then go from Squarespace back to Booqable, the _gl & _ga values persist.

Has anyone been successful setting up something similar? Or have any suggestions on what to do next?

Cheers,
Dan