Just add the "old" jtds-1.3.1.jar as a third-party JDBC driver. That adds SQLServer to the Database Type drop-down list for me on 19.1.
I do not use SQL Server, but here is recent discussion (looks like for SQL Server 2016) where jtds seemed to do the job:
We only support the jTDS driver today.
We're looking into the msft driver for easier SQL Server migrations.