Dear PavanReddy ,
You can do it on /oauth/manager by editing the oauth client key,
Note that the Custom Field needs to be json format.
when you validate the assess token with OTK Require Oauth 2.0 Token assertion, you can get the saved custom field from ${session.custom} as below,
{ "otk": {"client_type": "confidential","grant_type": "refresh_token"}, "portal": {}, "mag": {}, "clientkey": { Your custom field } }
Regards,
Mark