Ubuntu 18.4 : How to choose the sudo-account to use on "Authentication Required"?

When we need admin rights to do some stuff on Ubuntu 18.04, we are prompted with an Authentication Required window, asking for a sudo-account's password.

My problem is that it seems we can't choose which sudo-account we want to use.

This is a problem because my users are supposed to use a specific sudo-account to install some softwares, but the only one they can see when prompted for sudo-account, is the IT-account.

Is it possible/How to have the drop-down menu like there was in Ubuntu 16.04 ?

PS: I know this is a duplicate of this, but there is no answer for months.

Edit: Ok, let's assume that a drop-down menu is not possible. Is there a way to change the default account which appears ?