Hi HimanshuNankani-3787,
Thanks to post question here, first i want to say you that i am not very familiar with REST API but i have make some research for you.
First maybe you can create / update Registration Token by powershell ?
Here some informations about that : https://learn.microsoft.com/en-us/powershell/module/az.desktopvirtualization/new-azwvdregistrationinfo?view=azps-7.5.0#example-1-create-a-windows-virtual-desktop-registration-token
After that, i found this for REST API part : https://medium.com/wortell/getting-started-with-rest-api-for-wvd-arm-spring-release-and-sharing-my-github-examples-39593acb4c0e
Hope with all this informations, you can do what you need :)
Perion Cédric
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