Page which will display New Milestone popup in the org, there should be one button for Add new milestone. On click of this button, popup will ask for milestone details and after click on save button this new milestone details will add to DB.
Milestone is a custom object.
Create lwc component target for lightning__RecordAction. You can place your your code there.
If Save is not working, feel free the paste your error message, we can help better ?
If you want to make your custom popup, see below link https://www.infallibletechie.com/2020/03/popup-using-lightning-web-component-lwc.html