Getting references after a company closes - what worked for you?

The company closed and I am not sure how to reach former colleagues. I have read a lot of threads here and could not find my exact situation. HR only confirms titles and dates. How do you ask politely?

A client reference worked well for me. It shows you deliver outside the company too.

Ask former colleagues and supervisors in a short message and give them a summary of the job you are applying for.