Hello and welcome to our forums,
To achieve what you want you could add on a previous dialog (before the SQLConnectionDlg dialog) on "Dialogs" page a check box control event which will control the DB installation behavior. When the checkbox is checked its associated property (you can find it on "Properties" right pane after selecting the checkbox control) will be set to a text value, otherwise its property won't be set.
So, you can use the checkbox property to condition the display of SQLConnectionDlg dialog by using the
Show only if support. And also to condition the establish operation during install of the SQL Connection added in "SQL Databases" page. You should just add the "CHECKBOX_PROP" property in the "Condition" field.
All the best,
Daniel