pawelkb, if you’re a Virtualmin Pro customer, you’re welcome to file a support ticket using the Support link above. However, I’ll try to offer some advice below which should hopefully resolve that.
Also, note that the case of a MySQL password being set before Virtualmin is installed will be handled better in the next Virtualmin version.
But as for handling that now –
You should be able to go into Webmin -> Servers -> MySQL, click the gear icon on the top-left (or click Module Config, depending on your theme), and in there, you can set the “Administration login” and “Administration password” options.
Once you do that, you should then be able to re-run the post-install wizard by going into Virtualmin, and looking in the System Settings menu. If it doesn’t show up immediately, you may first need to run the Re-Check Config option.