Found the blog about adding button to pos using Owl, although its a odoo14 Version. but you can use it on odoo15 
the different is. you need to inherit the JS on manifest. instead of XML
https://www.cybrosys.com/blog/how-to-add-buttons-in-pos-using-owlhttps://www.cybrosys.com/blog/how-to-add-buttons-in-pos-using-owl