Here's a quick tutorial on how to create a custom prompt from Copilot Studio
Sign in to Copilot Studio
From the left nav, select + Create
Select New Microsoft Copilot extension
Select Copilot for Microsoft 365
Select New action
Select prompt
Enter a valid prompt name
You can now start to create a prompt
You have successfully created a custom prompt from Copilot Studio
Documentation
Create a prompt | Microsoft Learn
