Hi @Raghunandan Venkateswaran ,
Thanks for using Microsoft Q&A !!
Is there any specific reason you are trying to store the token to a file.
You can instead get the token using one Web Activity and then use token from the activity output to another Web Activity or use Copy Data with Http linked service for your GET method.
I do not have any Onelogin API to share you as example but you see below ADF where I have used Web Activity with Copy Data (copy data from rest API to a file on storage)-
Please let me know if you have any questions.
Thanks
Saurabh