When non registered click on ask question button, i want to open a popup or redirect to login page

Question

I want the non-registered user to ask the question, and fill all the fields and as he clicks on the Ask the question button. He should be redirected to login page or open the pop up for signup.

Any help on this highly appreciated. 

Comments

You could turn on the option to ask questions only if you're registered.

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Yup, that could be done but I want the non-registered to fill the complete the entire question form and then when he click the ask the question then I could ask him to signup or login.
I want to do this to increase user engagement coz user are less likely to abandon his efforts for filling question form.