You can try using item link with a search parameter for the selected text. e.g. x-devonthink-item://D633F2C7-1F40-4749-8BC0-000000000000?search=this%20is%20an%20example points to the first occurrence of the string this is an example in the document. You can write a script to generate this link with a keyboard shortcut.
Obviously there would be issues when the selected text appears more than once. This method should work most of the time, however, if you always select full sentences.