1. First of all, we have enable the “Secure Store Service” from the Central Admin:

Form Central Admin ==> Services on Server ==> start the Secure Store Service. 

2. Create  new secure Store Target application:

Form Central Admin ==>  Application Management  ==>  Manage Service Applications

==> Then select the “Secure Store Service Application" => Click Manage form the ribbon menu.

 3. Create new Secure Store for Target Application To set our credential on SharePoint

So insert any name for Target Application ID, Contact E-mail and we will make the type to be Individual as snapshot below:

Click next to set the fields of credentials  4. Set the credentials field type base the target application property So I’m going to change the filed to be User Name and Password For our external SQL server 

 5. Click Next to set the Target Application Administrator

Click OK  to save.
Then you’ll find the new Target Application Secure Store ID been created as below: 

 6. Set the User Name and password for External SQL and the Credential Owner by click on "Set Button" :

Then Save. 

On this stage we have our External SQL credential Stored on SharePoint Farm.nd the Credential Owner by click on "Set Button" :

Then Save. 

On this stage we have our External SQL credential Stored on SharePoint Farm. 

On SharePoint Designer level :

1. Create new external content type.

 2. Then let’s discover our external Content type.
 

3. Add connection and select SQL type: 

Now it is the Showtime:

In connection details we will use our Applications ID which we crated in pervious steps and saved on SharePoint to as Impersonated Custom Identity via Secure Store Applications.