Spark plugs are one of the quiet parts of a car. They fire every time the engine runs, and they wear a little with each use. Nothing on the dashboard announces when they are past their useful life, which is exactly why they get missed.
Start with the manual, not memory
Your owner's manual, or the maintenance schedule printed with it, lists when the plugs should be inspected or replaced. Intervals differ by engine, and some modern engines use plugs designed to last much longer than older ones. Do not rely on what a friend said or what the last shop guessed. Find the printed schedule for your exact vehicle and write the interval down on paper.
Know what your contract asks
Many service contracts require the vehicle to be maintained according to the manufacturer's schedule, and some ask for proof when a claim is filed. Read the maintenance clause in your own contract. Look for phrases like maintenance records, proof of service, or upkeep requirement. Whether plug work is named specifically varies from contract to contract, so treat all scheduled maintenance as if it matters and keep the paper that shows it was done.
Assign the job
Decide who books the work. If you do it yourself, your record is your own note: the day, the mileage, what was done, and what parts were used. If a shop does it, ask for an invoice that names the spark plug service and the mileage at the visit. An invoice that says tune-up without naming the plugs may not give you the record you need later.
Log it where you can find it
Keep the maintenance file with the contract, not in the glovebox of a car you might sell. Note the mileage at each change and the mileage when the next one is due. If you move, switch vehicles, or hand the contract to someone else, the file goes with the paperwork.
When you compare providers, look for which ones publish a sample contract you can read before you buy. The list is here: vehicle service contract companies that post a sample contract you can read first.