When making posts on your Google My Business listings, you have two options to interlink your posts. The first way is to simply click the 'Insert Previous Post SERP URL' button.
The second way requires you to have a Google My Business website.
Go to your GMB website and click into the post you wish to interlink out to. Take the URL of that specific post and put it onto the bottom of your latest GMB post.
For any other questions, please reach out to customer support by clicking the chat box at the bottom right of the screen.