Setting a default value in a drop-down list

Hello! I have a drop-down list that when a person's first name is selected then it will fill in the person's middle and last name. Is it possible to use a name that is frequently used as the default so that it is always filled in on the form? More specifically, how will it work so that the other fields (middle name and last name) will also be filled in with the default first name? Or is this not possible to do?

Thank you in advance!

Setting a default value in a drop-down list