Can I force users to a group based on the url they access for example?

You can add URL as a targeting attribute. Did you want that to persist? For example, if someone visits a specific URL, then they will always be forced a specific value even if they later navigate away from that URL?

We want to do an a/b test that the split is not done in growthbook, it is before in an ad for example. Then the users will either access link A or B and if it is link A they are control if they come from link B they are variation 1 for example

I see. So when the page loads, you just need to record which version the user saw so it can be analyzed in GrowthBook, right? If that's the case, you don't need our SDK at all and can just fire a tracking event on page load with your analytics system

That is true, I could trigger the ab_test_started myself