You don’t need to make any special sign up.
The version v2 refers to a particular header that must be sent, and there is only one working version of it.
Any “3rd party apps” - if they support OpenAI’s assistants API platform and endpoints - should and would be using the current version of “v2” already as would openai libraries behind the scenes.
Typical OpenAI-calling applications would use Chat Completions however, NOT the multi-call server-side-history Assistants.
Then: don’t provide your API keys to any untrusted parties, any website with “your API key here”.