Workaround:
Recently tried to mimic a similar task with URL PLUS field. The parameters works fine... at first sight:
- name field label "Youtube Channel"
- Set URL to standard Youtube www adress
- choose / as delimiter
- set VAL1 to second part: user
- set and use VAL2 for user to edit: YES
- you might set Description for additional help
- set Title to something meaningful like "Watch my cool YT channel..."
- set Edit Mode... how the user input will show-up in frontend submission.
- set Open link in ShadowBox (only alternative to the missing preview)
- Direct link should be o.k.
Unfortunately, the field params did not work/save correctly on my system... so, shadowbox did not open my external link. Maybe you or someone else can repeat the steps... I always had good experiences with this lightbox type! Could be a bug in my local setup...