can anyone help me out designing multitenant application plateform, where every tenant can have their user management, also how would be design a form where tenant can create custom field at run time.
There are a couple options depending how you want separate users and data. Others will know it better, but from what I understand you can have permissions, where certain data is only view able by certain users (like a Facebook or many online marketplaces). Or, which I have a feeling you want, you can have a main app (Parent App) and multiple sub-apps - the apps and their functionalities are the same but the databases are separate from one another (like a Salesforce).
We may need some more detail to what you are thinking of to help any more
Fore more info on the sub-apps you can check out this thread Allow creating multiple domains or child applications from a single master application