The first thing to do is to sign up with Musement if you haven't done so yet. You need an existing Musement account with published/ active products in order to connect.
Whats needs to be done in Bókun ?
Navigate to "Contracts" > "Online Travel Agencies (OTAs)" and select Musement.
Select "Add as reseller" up in the right hand corner.
You can now send Musement a Marketplace contract proposal including the products you'd like them to sell for you.