I have not used any Juniper kit before so my answer will be for the Cisco side.
Firstly you need to make the port a Trunk port.
(Vlan Management > Interface Settings)
Then you set the native vlan to be untagged.
(Vlan Management > Port to Vlan)
Then you tag any Vlans which you want to allow across the trunk.
(Vlan Management > Port to Vlan)
It literally is as easy at that,