Firstly you have to create the Bearer key from the envato API. Check for the step by step method. https://build.envato.com/api/#oauth
Finally with this step you’ll get the bearer key:
This bearer key will only valid for one hours. You can save your REFRESH_TOKEN for later use.
Every one hour later if you want to check then you will need to send a POST request again and then with the bearer key again send it to your envato api. Create an app for CLIENT_ID and CLIENT_SECRET. Save those infos also.
I am also trying to make an app for envato sale API. Hope it will be done soon. I can then send you the files.