When using YAML for builds, I could not find how to give an instruction that refers to the variable group I want to use. I had to manually open my build definition in the VSTS builds UI and create the link.
Does anyone know how to refer the variable group through the YAML?
Unfortunately we cannot link variable group through YAML for now. There is a user voice here submitted to suggest the similar feature, you can go and vote it up or submit a new one to suggest that again to achieve that in future release.
You have to link a variable group to the YAML build pipeline from the VSTS portal. Please see Use a variable group for details.
Another similar thread for your reference: How to access VSTS Variable Groups from YAML build pipeline?