For an employee to give feedback you need to ensure two things:
No negative repercussions
Promise that something is going to happen
Nobody will do the effort to give feedback if nothing is going to happen anyway.
How to ask for feedback depends on the workforce / audience and the goal.
Desk-employees can be reached easily via email-surveys and 1:1 meetings.
Blue-collar and service staff are harder to reach. Shift-start meetings can be the right forum or mobile feedback-apps where employees can submit feedback anytime and anywhere could suit your workforce.
The question itself should be adjusted for the audience. Classic questionnaires require time to study the questions.
Open-ended spoken feedback are better for spontaneous input for people not usually working at a desk.
Use the tool that supports the channels and question-format for your workforce.