Hi Jonashelp,
We see you're adding a second line to your dual-SIM iPhone and want to set up voicemail on that secondary line.
If you're prompted to create a password for your secondary line voicemail, this is expected as part of the voicemail setup. Once you've created a password, you would be able to complete setting up the voicemail for that line.
As to where, in the Phone app, when you tap on Voicemail for that second line, you should be prompted to set it up and then provided a box in which to enter the voicemail password. See more info here:
If you have further questions, feel free to reach back out. Additionally, let us know if this is an iPhone with an eSIM or two physical nano-SIM cards for the phone lines.
Cheers!