I'd suggest a slightly different approach: It might be more interesting for the reader of your code to know which API is being accessed than that it is an API. So if you're accessing calendar events or booking information through that API, you'd name the field calendarEndpointCalendarEndpoint or bookingEndpointBookingEndpoint.
The .NET naming guidelines referenced in another answer explicitly recommend against hungarian notation and using acronyms in identifiers, so this would also indicate that a semantically meaningful name is preferable to a technical name.