You can just create a table "connection(strings)" in your 1st db and create a model for that table.
After that you can do something like this:
$selected = Connection::query()->where('name', 'fromdropdown')->first();
$connection = $selected;
config(['database.connections.data' => array(
'driver' => 'sqlsrv',
'host' => $connection['Database_Server'],
'database' => $connection['Database_Name'],
'username' => $connection['Database_User'],
'password' => $connection['Database_Pass']
)]);
DB::setDefaultConnection('data');
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