There are two things you can do.
1. Tell them that you are applying in one year and would like a letter. (get confirmation that they would be willing to write it next year) Until next year, keep in contact with them and update them on your activities. Then, when you're ready to apply, ask them to write it.
2. You can use interfolio and they can write the letter now, have it stored, then when you're applying, see if they can re-submit the letter (they should have it saved), but with an updated date, because you want your letters to be dated the year you apply.