Retrieve the list of posts for a particular forum
To get all posts (forum topics) for a particular forum with id=4: 

curl -i -X POST \
  -H Cookie:SESSe6a3baadbeef=ovQpO6aYeehaw_3W4onya22i9bonusQW \
  -H X-CSRF-Token:v6j7r9RactujA872hSHSKW-mYYWAA \
  -H Accept:application/json \
  -H Content-Type:application/json \
  http://myserver/rest/taxonomy_term/selectNodes \
  -d '{"tid":4}'


To find the mapping between forums and tids, refer to this answer. 

I don't know how to do a parameterized query (GET) to retrieve forum topics. As far as I can tell you need to use POST, as described here.