Index to bookmark in topic

I'm using RH2015

I have a fairly long topic. About 3/4 of way down is a topic bookmark called "Sales Tax Calculation". If I click on the TOC entry in HTML 5 online help, it takes me to that location in the page.

If I create an index entry, it takes me to the start of the page and user has to scroll way down. If I select Cross reference and select the bookmark (not sure if this is the right hing to do). the index entry disappears when i regenerate

so how do I get the index to take the user to a specific location in the page/topic?

TIA

Mark