Clarification on LLM Options in "Create an Agent" Endpoint

In the "Create an Agent" endpoint, I noticed that the LLM field allows selecting between "OpenAI" and "Custom." From what I understand, the "Custom" option is mainly used to customize prompts—am I correct?

Also, I’m wondering if it’s possible to use my own OpenAI API key (for example, to access GPT-4.0 instead of 3.5) when choosing the "OpenAI" option. Would this be supported?