aylin duyguvar

I think the most important thing here is "anonymity". People should be ensured that their names would not be reported to the executive. I mean people should trust you.
Subordinates may submit online forms, and at the end of the feedback period (It may be one week at the end of the year) all the forms are sent to the executive.
But firstly you should provide a structured form which includes:

Feedback dimensions for the executive(e.g. interpersonal relationshis, leadership style, and else)

Employees should be given information about giving effective feedback at the beginning of the form. (You may attach a small video for this, so that people will be more eager to learn about giving effective feedback)

You may include some open ended items about strengths and weaknesses of the executive. (This may be like a checklist also)

I hope these help you to create your own answer to the question.
Kind regards