In order to send a message to users who received or will be receiving a specific campaign (Campaign A), you have to build a new campaign (Campaign B) and create a reference to the dispatched message from the preceding campaign (Campaign A).
Create a real time campaign.
Click on Add an event in User Journey section in Target group Tab.
When selecting an event, look for a "send"-related event from Campaign A (e.g., mail.send, fb-ca.sent, etc.) - namely, the event of sending a message in Campaign A.
Underneath, in the Event Properties section click on Add new and from the Property dropdown choose campaignId and select the ID of campaign you refer to (ID of Campaign A). Set the Operator to "equals".
Mark the option "this event needs to happen" within Event occurrence and set up the timing as per your campaign logic.
Click on Save event.
Add wait time as a second step of the user journey by clicking on Add a wait after this.
Select the required wait time as a time phrase and specify that it should come after the first step of the user journey.
Click on Save wait.
Proceed with building a message and preparing the new campaign for the dispatch.