Please follow the below instructions:

Step 1. Create a domain account which has the write permission to your UNC share

        Username: yourdomain/admin
        password="admin"

To access resources on a UNC, your application must run under a domain enabled primary secuirty token, the easiest way to do this is to use impersonation in your web config:

<identity
impersonate="true"
userName="yourdomain/admin"
password="admin"
/>

where the user name is a domain account with permission to the resouce. all users of your web site will have the same access permissions to the network resource.

Step 2. create a virtual directory on your Web server and point it to your UNC share folder.