Never mind, got it to work. Thanks a lot for the workflow!
For those who are wondering how to do it, edit the shorten script in the workflow, find & replace:
'bit.ly' : {'api_url':'https://api-ssl.bitly.com/v3/shorten?format=json&login=yourusername&apiKey=yourlegacyapikey&longUrl=','title':'bit.ly','des':'http://targetdomain.com'},
Target domain must be added to your 'custom short domain name' in bit.ly settings.