RE: Add button on card form Dynamics 365
Hi,
I was just struggling with the same problem.
What I've found to work best is adding the 'Mscrm.ShowOnGridAndQuickAction' enable rule to your custom button in the XRM Toolbox.
Also make sure that the rule's value is set to 'True'

Position of a button may vary, but for me adding it onto the 'Home' ribbon worked just fine.
Hope this saves anyone at least some time.
Cheers!