How do I enter a strength activity based on time?

This is one of those exercises that falls into the gray area. Gyminee currently only supports 2 exercise types, "Strength" and "Cardio." The distinction Gyminee uses is that strength exercises are tracked using sets, reps, and weight, while cardio exercises are tracked using time, distance, and a bunch of other common cardio-type things.

All of that is really to say that plank holds don't fit well into either. We chose to keep it as a strength exercise, because many people will perform multiple reps of them. If you would like to track the time metric, your options would be to re-create it as a custom exercise in one of your workouts, or you could also add notes to each performance to keep up with your time.