It'll be cool if you have more details about it :)
Are you a e-commerce, SaaS solution, service provider?
For each one, there're some approaches for it.
Besides, there's also the right moment to ask for it.
For example: If you sell products, you can use SurveyMonkey to ask for feedback some days after the user receives the product.
If you're a Saas solution, you can ask for feedback:
1-) Right after the user converts in your solution
2-) You can ask via call and ask for their permission to use.
It all depends from your solution and moment for using it :)