On the right hand side there should be a url next to Endpoint. It should look something like: redshift-warehouse.abcdef123456.us-east-1.redshift.amazonaws.com:5439/dev
The Host would be: redshift-warehouse.abcdef123456.us-east-1.redshift.amazonaws.com
From the Definite homescreen Settings Integrations Redshift, from there you should see the following screen in which you can input your Redshift credentials: