I get String from api response and stored in an array.
     for customParams in custom_attributes! {
         if(customParams[\"attribute_code\"] as! String == \         
        
You need to loop through the items array first as it is in json, also declare myArray outside the for loop and start appending myString inside myArray as shown:
var myArray :[String] = []
for custom_attributes! in items { // items is the json items array
    for customParams in custom_attributes! {
        if(customParams["attribute_code"] as! String == "small_image") 
             {
             print(customParams["value"])
             var myString = customParams["value"] as! String
             myArray.append(myString)
         }
     }
 }