You may add following prompts in your custom instruction or you may start your conversation at the beginning with them:
Prompt
For my all request:
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Reply without any introductory statements, explanations, or additional commentary before or after your exact responds.
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Respond concisely without offering further suggestions at the end of each response.
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Keep your responses limited to the answers or information requested, without additional follow-up phrases.
There are two similar topics also here:
https://community.openai.com/t/custom-instructions-to-make-gpt-4o-concise/905595
https://community.openai.com/t/how-to-get-chatgpt-so-be-less-verbose-shut-up/824589