To assign a post, you will first have to create a campaign, you can do this by going to Marketing > Campaign Overview and selecting Add new campaign. Once you have created a campaign, you will be able to assign posts.
There are two ways to associate your post to a campaign:
When composing your post
You can select the campaign you want to associate your post to when composing your post in the compose tab:
After your post has been published
You can also add posts to campaigns retrospectively by going to the Marketing module > Outbox and finding the post you'd like to add to a campaign. From here, hover over the three dots and select Add Campaign. A drop down menu will appear for you to choose from.
From the outbox, you can also change the campaign a post is associated to by hovering on the three dots and selecting change campaign.
The icon below will appear on posts in the outbox that are part of a campaign, you can hover over the icon to see which campaign they are in.