How to use subscribed data in another request in Angular

I have post request which will submit the data. Once the data is submitted, I get the response where it contains ID of the item submitted.

That ID I have assigned it to a variable.

Now, I need to use that variable in another post request.

When I'm trying , I can see value is blank

 this.sharepointService.createItem().subscribe( (response : Response) => {
      this.lastItemCreatedId = response['d'].ID;  // latitemcreated is a variable
      console.log("Newly created Item ID :" + this.lastItemCreatedId)
    })

Now I need to use lastItemCreatedId in another request

      let url = "_spPageContextInfo.webAbsoluteUrl"+"_api/web/lists/getByTitle('List')/items('"+this.lastItemCreatedId+"')/AttachmentFiles/add(FileName='abc.txt')"

 this.sharepointService.addAttachementstoItem(url).subscribe()

Info : Above two are post requests

How can I use lastItemCreatedID variable value in another post request ?