During a project ' s planning phase, the project team identifies a potential supplier delay...

A risk trigger is a specific event or condition that signals that a risk is about to occur or has occurred. PMBOK® Guide clarifies:

" Risk triggers (sometimes called warning signs) are indicators or symptoms that a risk event is about to occur. The risk register should include identified triggers for each risk. "

— PMBOK® Guide, 6th Edition, Section 11.2.3.1

Thus, documenting the specific event or condition (such as the supplier missing a delivery deadline) is the correct approach.