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 VS
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?