This topic gives you an example of how to make the EnableScalingGroup API request according to the scaling configuration you specified, and add an existing ECS instance.

http://ess.aliyuncs.com/?Action=EnableScalingGroup 
&ScalingGroupId=dP8VqSd9ENXPc0ciVmbcrBT1
&ActiveScalingConfigurationId=eOs27Kb0oXvQcUYjEGelJqUy
&InstanceId. 1=i-283vvyytn
&<Public Request Parameters>

Response example

< EnableScalingGroupResponse>
    <RequestId>6469DCD0-13AC-487E-85A0-CE4922908FDE</RequestId>
</ EnableScalingGroupResponse>