How to set a default signature for a channel
You must be an admin or have the permissions within Front to edit settings for a team signature.
Go into your Settings.
Click Signatures under your team.
Click on the signature you would like to set as default.
On the left, click Channels.
If you would like to restrict the signature to a specific channel, disable the Signature visible for all team channels option. Uncheck any channels that you don't want your signature to be available in.
Note: Make sure your individual signatures are not visible to that channel. If it is visible, this will end up being your default signature for that channel.
Also, see these steps on how to create or edit a team signature.