You can set the default landing page for Portal by using JS API as follows
TWUser.setAttributeValue("Portal Default Page" , {URL of Service / Dashboard})
You can also set it with Bulk Attribute Assignment in Process Admin Console,
or with REST API call
PUT /rest/bpm/wle/v1/user/{userNameOrID}?action={string}&key={string}&value={string}
where action will be setPreference and key will be "Portal Default Page"
and obviously the manual way is to click on the Plus icon when you are viewing the page/service/dashboard