Does anyone know how to specify the security groups and the subnets on the ECS Task?

Believe you have to JSON patch the "Task Customizations" field on the ECS Task block. I did this for subnet:

[{"op":"add","path":"/networkConfiguration/awsvpcConfiguration/subnets","value":["subnet-XXXXXXXXXXXXXXXX"]}]

Not sure about security groups, but if that's needed it could probably be added in a similar manner