How do I create campaigns from code with the option “Combine with all other campaigns” selected? If I get all campaigns and add with activeCampaign.CombineWithCampaigns, will that make sure that new campaigns also are added when created?
You need to add all existing campaigns as combined with on the new campaign.
The feature combine with all other campaigns is just UI added for selecting all campaigns this way. New campaigns are not automatically connected but if that functionality is needed please report it as a bug so that it will be investigated.