Q: Should I use the name of the first author or the corresponding author before "et al." in a reference?

You should always use the first author’s name before "et al." and not the  name of the contact author. This is because the first author is the lead author or main author of the paper; typically, the person who carried out the majority of the research with the assistance of the other co-authors. The contact author or corresponding author is usually a senior author who provides intellectual inputs and approves the protocols to be followed in the study.

Since the contribution of the first author is the maximum, the name of the first author should always be included in reference and citations, followed by "et al."

You might also be interested in reading this informative article about Using "et al." for in-text citations in research papers.