According to the Nielsen-Norman Group, marking both required and optional inputs makes form completion a bit easier.^[1]

The most common way of indicating a required field is by using an asterisk — the universal symbol to mark a mandatory input. Another way is to put the word "required" near the label. Avoid using muted grays or low-contrast colors as they may create an unnecessary challenge for people with visual impairments and older adults.

To indicate that a field is optional, simply include a note in the input label within brackets with the word “optional.”