Using promise with observable in ionic 2 applications

I am trying to call a web service in ionic 2 application but getting struck in between promise and observable. This is my function which i am calling on click of a button-

getUserDetailsIfAccessTokenIsSuccess()


{
    this.remedyService.userGetDetail(this.loginDetailsObj.username).subscribe(
      data =>{
        console.log("userGetDetail Success="+JSON.stringify(data));
      },
      err =>{
        console.log("userGetDetail Success="+JSON.stringify(error));
        },
       () =>{
              console.log("Get Userdetails Completed");
      });
}

Now i am calling userGetDetail function in my service file.

userGetDetail(eid){
    var url = '/userGetDetail';
    var params = {
                'EID':eid
            };
    console.log(JSON.stringify(params));
    var headers = new Headers();
    this.createAuthorizationHeader(headers);
    this.storage.get('remedyAccessToken').then((value)=>{
        var token  = value.toString();
        console.log("TOKEN IN REMEDYSERVICES="+token);
        headers.append('Authorization', token);
        console.log("header value inside="+JSON.stringify(headers));
    })
    console.log("header value outside="+JSON.stringify(headers));
    let options = new RequestOptions({ headers: headers });
    this.response = this.http.post(url,this.urlEncode(params),options).map(res => res.json());
    console.log("Response JSON="+JSON.stringify(this.response));
    return this.response;
}

Now the issue is that before Authorization is getting appended to header the flow is going to next line and web api is giving error. Is there anyway i am can synchronise in such a way that after Authorization is appended then only service call should happens. I tried putting the web api call inside then block but it makes the service calls as a promise. Any help will be appreciated.