Sharepoint Online - Set a user as SCA using Rest API

In order to set a user / Security group as a Site Collection administrator, we can use Rest API. 

Below is the Post call, with request body details

 //Rest URI  
 _api/web/siteusers  
 //headers  
 {  
   "Accept": "application/json; odata=verbose",  
   "content-type": "application/json; odata=verbose"  
 }  
 //Body  
 { "__metadata": { "type": "SP.User"},  
  "LoginName": "<<Login Name>>",  
  "IsSiteAdmin":"true"  
 }  

we are actually setting the IsSiteAdmin flag to true, if we want to remove SCA, we can set it to false.

Note: If the site already has this user, it will update the existing user