is there any way for predetermine the user and server for the client?
The client Server and Username values are storer in the Windows registry in this setting:
So you could create the desired Reg entries for importation for each user, prior to their 1st run of the SSLVPN client.
For use of the OpenVPN client instead of the WG SSLVPN client, the "remote" (no quotes) keyword line(s) in the downloaded client.ovpn file have the server IP addr(s) or domain name(s) included from whatever was entered in the SSLVPN setup on the firewall.
The User option is not supported for the Windows version of the OpenVPN client, at least when I test it.