How to buy more Azure DevOps pipelines build minutes? As mentioned on the Azure DevOps official pricing page, the plan provides 1 Free Microsoft-hosted CD/CI parallel job with up to 1800 minutes per month and 1 Free Self-hosted CD/CI parallel job with unlimited minutes per month. 

When we exceed the limit of Microsoft-hosted minutes, what I do is configure a VM as the build agent. How can I buy more build minutes after exceeding this 1800 minutes, rather than launching VMs for agents by myself?
You can set up billing in your Organization settings in DevOps

https://dev.azure.com/~your organization name~/_settings/billing