Welcome @cathyjliu! 
If I understand you correctly youâd like to pin a post within a topic. At the moment, this is not possible by default. Iâd imagine that you would need a custom plugin.
An alternative way to make a post stand out is to âAdd Staff Colourâ
This is found on the 
icon under a post along side like, bookmark etc
Although it does not âpinâ the post it makes it more visible.