How to create a Siri Shortcut that prompts to select a Reminders list that can be reused as a variable
This is a very hacky solution with annoying drawbacks.  I hope someone else comes up with something better.

The only way I found to set a variable to a user-selected list from the Reminders app is to add a dummy reminder that allows the user to set the list (and alert), from which you can extract the list (and alert) and store them for re-use into variables.  Then you just immediately delete the new dummy reminder.

The steps are:


Static text with the (unique) value "Dummy reminder"
Add New Reminder (Reminder: "your selected items", List: Ask Each Time, Alert: Ask Each Time)
Set variable "Selected reminders list" to (List from #2).  To do this, you have to tap the "List" and set "Get" to "List"
Set variable "Dummy reminder" to select magic variable: \New Reminder\
Set variable "The alert" to select magic variable: \Dummy Reminder\ and change "Get" to "Has Alarms"
Remove (select magic variable) \Dummy Reminder\


Then you can use these variables however you want:


Selected reminders list
The alert


The drawbacks:


While I entered a variable name "your selected items" to try and suggest that something will happen with the text you're running this shortcut on, the prompt to set the list and the alert is not very intuitive.  It reads "Add \your selected items\ to \default reminders list\ with \No Alert\".  You have to tap your default reminders list name in the text of the prompt to change it (as you do for the alert).
The user gets a non-specific warning that a reminder will be deleted and that it can't be undone, but they have no idea what it is.  It states "Remove 1 Reminder? This is a permanent action. Are you sure you want to remove this item? \Cancel\ \Remove\".
If the user doesn't tap or taps "No Alert" but doesn't enter a time (even though a grayed out default time is displayed), they will get an error that states "The alert time provided was invalid. Please check for spelling errors and ensure you provide a time of day."


Alternatively, you can do something similar using the "Find Reminders" method, but the prompt is even less clear/intuitive.