Hi @AbhishekBiswas ,
Forge does not currently support Atlassian OAuth. Here is a thread discussing a similar issue: Forge External Oauth2 doesn't support Atlassian Oauth? - #10 by ibuchanan . You may be able to get around this with API tokens.
For other external requests you will need to update your manifest to include relevant permissions: https://developer.atlassian.com/platform/forge/manifest-reference/permissions/#client