the numerator "1" is located above on a line and the denominator "1-x" is located under the line. In the other word, the fraction should be written in a vertical style.
Origin doesn't have a full-featured equation editor, so any solution might be a bit of a kludge. Here are two:
ln(\=(\ab(1-x), 1))
ln(\=(1-x,\u( 1 )))
(The text shown should be pasted into the Text Control box.) Of course, you can arrange text and lines anyway you like and then use the Group tool on the Object Edit toolbar to make one object out of all Shift-selected object.