How to set Pref.Source for dynamic routes?

RouterOS General
1

Hello,
How to set Pref.Source for dynamic routes type DAS (dynamic active static)? (for example pptp/l2tp/sstp)

2

Something should be possible with routing filters. Just to give you idea where to look:

http://forum.mikrotik.com/t/2-ips-and-preferred-source/123457/1

But I didn’t play with it much myself.