Try adding below in cmdb_ci dictionary override for incident table:

1. javascript:new getCI().getClassCI(current.subcategory);

2. Create Script include named getCI with below code:

getClassCI :  function(subcat){
 var arr = [];
 var ci = new GlideRecord('cmdb_ci');
if(subcat == 'hardware'){ // verify backend value of subcategory
ci.addQuery('sys_class_name' , 'cmdb_ci_computer'); 
ci.query();
while(ci.next()){
arr.push(ci.getUniqueValue());
}
}
if(subcat == 'database'){// verify backend value of subcategory
ci.addQuery('sys_class_name' , 'cmdb_ci_database'); 
ci.query();
while(ci.next()){
arr.push(ci.getUniqueValue());
}
}
if(subcat == 'application'){/ / verify backend value of subcategory
ci.addQuery('sys_class_name' , 'cmdb_ci_appl'); 
ci.query();
while(ci.next()){
arr.push(ci.getUniqueValue());
}
}
return arr;
}