When I add a portal on the User layout
Make sure that this is a portal to assignments, not Keys.
and a drop down menu to select a key number from the "Keys" table
Thus should be the assignments::KeysID field and the value list should list values from the Keys::KeysID field. A name or description field from keys can be listed as the second field to make it easier to select the correct KeyID value for the new join record this action will create. You can add fields from the keys table to this portal and they will display data from the selected record when you select a value from the drop down list.
Here's a demo file where the basic set up layout is set up along the lines just described. The additional layouts illustrate more sophisticated approaches that can also be implemented when working with a many to many relationship:
Thank you so much for your help! It worked! Awesome!