How do I flag 'I did it'?

Morning all, I am currently in week 4 of the Dirt Destroyer training plan and I have an optional unstructured ride which I wont be doing as I will be doing an IRL ride as suggested in the plan. It reminds me to mark it as ‘I did it’ but I don’t know how to do this? Sorry if it’s a dumb question but how do I tell the plan that ‘I did it’ ?

Thanks in advance

There’s a tiny “I DID IT!” link in the bottom right of the workout panel. Just by where it says “49 TSS”.