If you enable automatic deployment for a dedicated host, the dedicated host is added to the resource pool of your Alibaba Cloud account for automatic deployment. When you create an Elastic Compute Service (ECS) instance, you do not need to specify a dedicated host. The system selects a dedicated host from the resource pool to deploy the instance. This improves deployment flexibility.
By default, Allow Automatic Deployment is selected when you create a dedicated host. Unless you manually deselect this option, there is no need to enable it again.
Procedure
Log on to the ECS console.
In the left-side navigation pane, choose Instances & Images >
> Dedicated Hosts. In the top navigation bar, select the region and resource group to which the resource belongs.
Select the dedicated host that you want to manage. In the upper-left corner of the Host tab, choose
> Modify DDH Info. 
In the Modify DDH Info dialog box, switch on Automatic Deployment.
Click OK.
References
You can also call ModifyDedicatedHostAttribute to add your dedicated host to the resource pool for automatic deployment.
Alibaba Cloud provides the following methods to deploy an ECS instance on a dedicated host:
Enable automatic deployment. The system then selects a dedicated host from the resource pool to deploy the instance. To do this, when you create the instance, select Random Dedicated Host (Automatic Deployment) / AutoPlacement for the Dedicated Host parameter in the Advanced Settings (Optional) section. For more information, see Create an instance on the Custom Launch tab.
Specify a dedicated host when you create the instance. For more information, see Create ECS instances.