Issue
I have Granted Access to Allow Other Team Members to Send from a Shared Mail Box.
How Do I Use the Senders Field in a Signature Configuration?
Solution
1. Create a Formula Field
a. From the Configuration Database Go to Admin > Fields

b. From the Menu Click on CREATE NEW

c. From the Drop Down Menu Select Formula

d. Below are the Formula Field settings:
Field Name: OnBehalfOf
Exclude from Cache: Yes
Exclude from Cache: Yes
Formula: Copy and paste the code attached into the editor

2. Create a Field which Looks up the Formula Field
a. From the Configuration Database Go to Admin > Fields

b. From the Menu Click on CREATE NEW

c. From the Drop Down Menu Select Field (from Active Directory)

d. Below are the Field Settings:
Field Name: SenderDisplayName
Field Type: Active Directory Field
AD Attribute: displayName
Lookup Key Type: Use Another Field
Lookup Key: OnBehalfOf (From the Drop Down List is the field created above)
Text List separator: Carriage Return - Line Feed
Exclude from Cache: Yes
Field Type: Active Directory Field
AD Attribute: displayName
Lookup Key Type: Use Another Field
Lookup Key: OnBehalfOf (From the Drop Down List is the field created above)
Text List separator: Carriage Return - Line Feed
Exclude from Cache: Yes

3. Add the New Field to Your Signature Configuration

4. Send a Test Message from the Shared Mailbox
Below: The Composed Message

Below: The Delivered Message in the Recipients Mailbox with the Senders Display Name

You can find the code here: