You have 2 ways :
1) if you are using Odoo EE and you are not a developer: 
Install Odoo Studio and Open the Studio Mode in your form and click on the field to select it and then an option will appear on the left called "Required" check it and save.

2) If you are using Odoo EE or CE and you are a developer:
Activate the developer mode from Odoo Settings and then go to your form and click on the debug icon on the top right corner and select the "Edit Form" menu option you will see an xml code search for your field and type required="True" and save the view and then refresh. 

Happy to help :) an upvote will be awesome