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Let's assume the text field is called "Notes". Create a formula text field with the following formula:
Left([Notes], 100) &
" " & URLRoot() & "db/" &
[Record ID#] &
where the number eleven above is the identifier of the custom form that displays only the field "Notes". To determine the dfid of your custom form please read:
The above formula displays the first 100 characters of the "Notes" field. To dipslay more or less characters change the number one hundred above.