ARSetupMaint.cs

using PX.Data;

using System.Collections;
using System.Net.Http;
using System.Threading;

namespace GetValueFromAPIExample
{
    public class ARSetupMaint_Extension : PXGraphExtension<PX.Objects.AR.ARSetupMaint>
    {
        // we need IsActive here to be able to enable/disable the extension conditionally. 
        // for more information see https://help.acumatica.com/Help?ScreenId=ShowWiki&pageid=cd70b408-b389-4bd8-8502-3d9c12b11112
        public static bool IsActive()
        {
            return true;
        }

        [InjectDependency]
        // IHttpClientFactory is used to create HttpClient instances efficiently. 
        // It helps manage the lifetime of HttpClient, preventing common issues like 
        // socket exhaustion caused by instantiating HttpClient manually.
        public IHttpClientFactory HttpClientFactory
        {
            get;
            set;
        }

        // this adds a button to the ARSetup screen
        public PXAction<PX.Objects.AR.ARSetup> GetDataFromExternalAPI;
        [PXButton(CommitChanges = true)]
        [PXUIField(DisplayName = "Get Data From External API")]
        protected IEnumerable getDataFromExternalAPI(PXAdapter adapter)
        {
            string responseBody;

			using (var client = HttpClientFactory.CreateClient())
            {
				// this is a special way to run async operations in Acumatica
				responseBody = Base.LongOperationManager.Await(async (CancellationToken token) =>
                {
                    HttpResponseMessage response = await client.GetAsync("https://reqres.in/api/users", token);
                    response.EnsureSuccessStatusCode();

                    return await response.Content.ReadAsStringAsync();
                });
            }

            // since the custom field we want to write the data to is defined in an extension, we need to get the extension object first
            var extension = PXCache<PX.Objects.AR.ARSetup>.GetExtension<ARSetupExt>(Base.ARSetupRecord.Current);
            extension.UsrTestField = responseBody;

            //need to update the record for the changes to be properly applied
            Base.ARSetupRecord.Update(Base.ARSetupRecord.Current);

            return adapter.Get();
        }
    }
}