Adding Followup Actions 
You can add followup actions to message log actions and static method call actions.
In the Object Manager panel, under the General section, choose the Followup Actions tab.
The Followup Actions tab page opens.
You can add a new action or add an existing action to the message log action.
Add a New Action
Choose Add new Action.
In the Object Creation dialog box that appears create a new action.
More information: Creating an Action Type
Add an Existing Action
Choose Add existing Action.
In the Object Query dialog box that appears, select the action and choose Select.
The selected action is added as a followup action.
Save the action.