Want to Make A Custom Field As Mandatory on Lead Qualify Button Click

Hi All,

I have requirement where i want to make a custom field called lead qualification comments as mandatory before user click on qualify lead button.

Can anyone please let me know how can we achieve this? As per initial requirement it is nice we can have pop up for the users to give a pop up to give the comments how ever it is also fine if we make this as mandatory with message lead qualification comments are mandatory.