From : Allowlisting external applications

• Allowlist the consumer key.
  1. Stop Confluence.
  2. Add the com.atlassian.ratelimiting.whitelisted-oauth-consumers system property, and set the value to a comma-separated list of consumer keys, for example: 

     -Dcom.atlassian.ratelimiting.whitelisted-oauth-consumers=app-connector-for-confluence-server
     

     The way you add system properties depends on how you run Confluence. See Configuring System Properties for more information.

  3. Restart Confluence.

com.atlassian.ratelimiting.whitelisted-oauth-consumer seems to only support OAuth 1.0a application links

Workaround
If external application support OAuth1, then create an OAuth 1.0a link to the external application by selecting Atlassian product instead of External application during the application link creation screen. Selecting Continue after the warning screen will provide the remaining options for creating an OAuth 1.0a link.