Requirement for creating an auto number field is very common & achieving them used to be very difficult or unsupported(using third party tools) . Now with Dynamics CRM 365, you can create an auto-number attribute very easily.

At this moment its not available in the UI , you will have to do it programmatically but its  very less and easy code.

following the format which can be achieved :

lets see how this can be done , in this example i am giving my custom organization entity a serial auto numbered field. you can do this both by C# code or Webapi :

C#

CreateAttributeRequest widgetSerialNumberAttributeRequest = new CreateAttributeRequest

            {
             EntityName = "new_organization",
             Attribute = new StringAttributeMetadata
                {

                    //Define the format of the attribute

                    AutoNumberFormat = "ORG-{SEQNUM:3}-{RANDSTRING:6} ",
                    LogicalName = "new_serialnumber",
                    SchemaName = "new_SerialNumber",
                    RequiredLevel = new AttributeRequiredLevelManagedProperty(AttributeRequiredLevel.None),
                    MaxLength = 100, // The MaxLength defined for the string attribute must be greater than the length of the AutoNumberFormat value, that is, it should be able to fit in the generated value.
                    DisplayName = new Microsoft.Xrm.Sdk.Label("Serial Number", 1033),
                    Description = new Microsoft.Xrm.Sdk.Label("Serial Number of the widget.", 1033)
                }

            };
            crmService.Execute(widgetSerialNumberAttributeRequest);

As you can see in my request i have given AutoNumberFormat  as ORG-{SEQNUM:3}-{RANDSTRING:6} from the List.

in case you need the full code:

using System;

using System.ServiceModel;

using Microsoft.Xrm.Sdk;

using Microsoft.Xrm.Sdk.Metadata;

using Microsoft.Xrm.Sdk.Messages;



amespace Microsoft.Crm.Sdk.Samples

    {

        public class FollowupPlugin : IPlugin

        {
            public void Execute(IServiceProvider serviceProvider)
            {
                //Extract the tracing service for use in debugging sandboxed plug-ins.
                ITracingService tracingService =

                    (ITracingService)serviceProvider.GetService(typeof(ITracingService));
               // Obtain the execution context from the service provider.

                IPluginExecutionContext context = (IPluginExecutionContext)
                    serviceProvider.GetService(typeof(IPluginExecutionContext));
            

            CreateAttributeRequest widgetSerialNumberAttributeRequest = new CreateAttributeRequest
            {

                EntityName = "new_organization",
                Attribute = new StringAttributeMetadata

                {
                    //Define the format of the attribute
                    AutoNumberFormat = "ORG-{SEQNUM:3}-{RANDSTRING:6}  ",
                    LogicalName = "new_serialnumber",
                    SchemaName = "new_SerialNumber",
                    RequiredLevel = new AttributeRequiredLevelManagedProperty(AttributeRequiredLevel.None),
                    MaxLength = 100, // The MaxLength defined for the string attribute must be greater than the length of the AutoNumberFormat value, that is, it should be able to fit in the generated value.
                    DisplayName = new Microsoft.Xrm.Sdk.Label("Serial Number", 1033),
                    Description = new Microsoft.Xrm.Sdk.Label("Serial Number of the widget.", 1033)
                }
            };

            crmService.Execute(widgetSerialNumberAttributeRequest);
       }

    }

WebAPI

{
 "AttributeType": "String",
 "AttributeTypeName": {
  "Value": "StringType"
 },

 "Description": {
  "@odata.type": "Microsoft.Dynamics.CRM.Label",
  "LocalizedLabels": [
   {
   "@odata.type": "Microsoft.Dynamics.CRM.LocalizedLabel",
    "Label": "Serial Number of the widget.",
   "LanguageCode": 1033
   }
  ]
 },

 "DisplayName": {
  "@odata.type": "Microsoft.Dynamics.CRM.Label",
  "LocalizedLabels": [
   {
    "@odata.type": "Microsoft.Dynamics.CRM.LocalizedLabel",
    "Label": "Serial Number",
    "LanguageCode": 1033
   }
  ]
 },

 "RequiredLevel": {
  "Value": "None",
  "CanBeChanged": true,
  "ManagedPropertyLogicalName": "canmodifyrequirementlevelsettings"
 },

 "SchemaName": "new_SerialNumber",
 "AutoNumberFormat": "WID-{SEQNUM:5}-{RANDSTRING:6}-{DATETIMEUTC:yyyyMMddhhmmss}",
 "@odata.type": "Microsoft.Dynamics.CRM.StringAttributeMetadata",
 "FormatName": {
  "Value": "Text"
},
 "MaxLength": 100
}

You just have to pass certain headers in the API request such as Content -Type , Application as JSON and the authentication.

Once you have done this – you will see a a new field as new_serialnumber has been created in the system under new entity. Few things:

1. Put the field on the form
2. it will be a locked field
3. it will generate a new number everytime you click on save

it will be something like below:

I hope this helps!