I'll recommend you take a look at HelpScout blog (https://www.helpscout.net/blog/knowledge-base-article/) and intercom blog (https://docs.intercom.io/). They're definitely the best resources to dive into this questions :)

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 :)