How to grant user rights to create stored procedures in a custom schema
You can't change the ability to create procedures to only one schema by only granting permissions on the schema. (Assuming the user has no other rights.)

Why?

The user still needs the right  to create objects in the database, which are in this case procedures.

What you can do, is grant the user CREATE PROCEDURE rights, and then either change the owner of the schema to that user (more secure, see below for more information) or grant that user permissions on the 'IC' SCHEMA. 

Only granting CREATE PROCEDURE does not allow the user to create procedures on  schema's such as dbo.

Create the login, the corresponding user and the IC schema

USE [master]
GO
CREATE LOGIN [TestIC] WITH PASSWORD=N'Test', DEFAULT_DATABASE=[master], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF;
GO
USE [test]
GO
CREATE USER [TestIC] FOR LOGIN [TestIC];
GO
CREATE SCHEMA IC;


Grant Create procedure to the user

GRANT CREATE PROCEDURE TO [TestIC];


Either Change the owner of the schema (Safer Option) 

ALTER AUTHORIZATION ON SCHEMA::IC TO [TestIC];


Or grant the user access to the schema

GRANT 
     ALTER

ON SCHEMA::IC


      TO [TestIC] ;


Risk involved with granting rights on the schema


  Granting a user the ability to alter another user's schema gives that
  user the ability to SELECT, INSERT, UPDATE and DELETE rows in any
  table owned by the owner of that schema. This is called "Ownership
  Chaining" and it's what makes Views and Stored Procedures really
  simple ways to control security, as a user who has permissions on a
  View or Stored Procedure does not need to be granted permissions on
  the underlying Table, so long as the View/Proc is has the same owner
  as the Table.


Source

Take a look at the answer by @DavidBrowne-Microsoft for the origin of the quote, and to get more information on "ownership chaining" security risks when granting alter schema rights.

Test

EXECUTE AS LOGIN  = 'TestIC';

CREATE PROCEDURE IC.test
as
select * from sys.databases; 


Result:


  Commands completed successfully.


This fails

CREATE PROCEDURE dbo.test
as
select * from sys.databases; 



  Msg 2760, Level 16, State 1, Procedure test, Line 1 [Batch Start Line
  35] The specified schema name "dbo" either does not exist or you do
  not have permission to use it.


Undo impersonation

REVERT;