Can Botox be used for under the chin? I don’t really have double chin.

Botox won't decrease the fullness or double chin under the neck. It can reduce some of the vertical platysmal muscle bands which are not present in many patients who have a double chin. Fat accumulation or loose skin under the neck is better treated with CoolSculpting, SculpSure, ThermiTight etc.

Thank you for your question.
Botox wouldn't really be helpful here.
Sounds like you may benefit from some tightening or fat reduction. How you do that is up to your personal preference. There are surgical or non-surgical methods. It sounds like your condition is mild to moderate, something like ultherapy or non-invasive methods of fat reduction may be something to consider.
Hope this helps.

No, I don't recommend Botox for this. Both Kybella or neck liposuction are good options and will improve the double chin. It depends on your lifestyle and personality which would be better for you. 
Kybella is best for patients that:
1)  Do not want surgery 
2)  Are not surgical candidates 
3)  Not willing to wear a compression dressing around the face for 1 week 
4)  Patient-  The results of Kybella will take months
Kybella take 1-3 treatments every 6 weeks.  Patient will develop swelling and possible bruising after each injection that may last a few days.  The benefit of Kybella is each treatment is quick and completed in the office. However, some patients need more than one treatment to get desired results. 
Neck Liposuction is best for patients that:
1)  Want a single treatment 
2)  One recovery time-  Downtime for neck liposuction require wearing a wrap around face for 1 week. 3)  Less painful 
4)  Quicker results 
5)  Cost is a concern
Neck liposuction is a quick surgical procedure that can be complete while the patient is awake.  It cost around the same price a Kybella and can be less in patients with heavier necks.  The single recovery of neck liposuction may be a better option for some patients.