You are actually doing this now whenever you ride free ride outdoors. 
Just not intentionally.
You may ride without any particular goal in mind or join a group ride. When you return and upload your activity Xert analyzes the ride and quantifies focus duration, difficulty, intensity (strain ratio), and strain score (XSS).
The difference is a free ride to focus entails monitoring a few fields to zero in on the target focus type using target watt intervals to maintain that focus point as you accumulate XSS for the day. IOW a solo ride with purpose. 
This video from Scott Steele demonstrates the process using Connect IQ data fields on a Garmin. I use EBC on one of my Android devices. There are differences between the two platforms but the fields to monitor are the same. Focus. TTE. and XSS are a default page view on EBC, but you can customize the field layout as you like.
Ride to Xert's Focus Metric with Scott - YouTube
Scott’s demonstration is indoors but he is simulating what you would do outdoors. It makes a lot more sense once you start doing it. 
Also note the video doesn’t show RIBs but of course they are part of the equation.
You control the outcome by monitoring watts (roughly at or above target for work intervals) and the focus field. What you end up with is a combination of these elements as your ride description on XO.
- Rating: Easy, Moderate, Difficult, Tough
- Specificity: Polar, Mixed, Pure (unlikely outdoors)
- Focus: 2 min (Road Sprinter), 3 min (Pursuiter), 4 min (Puncheur), 5 min (Breakaway Specialist), 6 min (Rouleur), 8 min (GC Specialist), 10 min (Climber), 20+ minutes (Endurance)
For example, my last outdoor ride to focus in December was “Tough Mixed Puncheur Ride” to around 4 minutes on my favorite 3+ hour loop. The same loop I’ve ridden to Moderate Polar Endurance (~18 minute focus) and Difficult Mixed Pursuiter (6 min) in the past.
In practice this means being careful how you start out on a ride. An endurance ride (20 minutes and higher) requires a level of concentration to spin light and easy regardless of the terrain. A strong start or a few out of the saddle efforts will drive focus down. It’s much harder to raise the Focus number than it is to drive it down. Well, that is until you get to low focus numbers which require harder intervals to drive the number down repeatedly.
Like anything practice makes a difference plus learning which routes (or route segments) are best for a range of targets. Some routes are ideal for harder workouts (lower focus number) while a flat course is required to remain light and easy (20:00+).
There are no specific interval durations or repeats to follow. You ride irregular intervals to attain and maintain your targets. Once you learn the pattern it’s fun to do. Endurance focus is actually the harder target to maintain. Even with 1:1 gearing a steep slope or strong wind can throw you off course as will surging at traffic lights. Keeping it easy takes on a whole new meaning.
Shifting and pedaling are key factors as the Focus field won’t change at all while coasting.
The Interval Target is a rough watt number to attain. You want to stay above it during work intervals, but you can also surge higher.
Your goal is to achieve the intended focus, strain ratio. and score for the day. It doesn’t have to be exact. Focus duration points are just that. Points along your power curve. You want to end up within range of the target. When you return and upload your activity, focus duration will change some based on the deeper analysis performed on the XO server. IE, how well and how often you maintained the targets during the ride.
Riding outdoors to focus is a learned skill. Once you understand the principals and method it becomes another wrench to grab from your training toolbox. I much prefer this option than trying to replicate a workout outdoors.
This also explains what XATA means on the left side of the Training tab page.
Confusing to newbies no doubt but Focus Type and Interval Target (watts) refer to free ride to focus outdoors versus the highly structured indoor workouts on the Recommended List.
While some workouts lend themselves to completion outdoors, riding to focus adds a whole other dimension to your rides. You can completely ignore the recommended list of workouts while attaining your goals within XATA guidelines.
Related post –
Estimate outdoor ride XSS and Focus - General - Xert Community Forum (xertonline.com)