This seems to have to do with your web page coding rather than Word.
Why not simply use a mapped Content Control in the Word document and let them enter the data there?

• Repeating Data Using Document Properties Content Controls and Other Mapped Content Controls
• Mapped Content Controls by Greg Maxey, MVP
• Insert Content Control Add-In by Graham Mayor, MVP
• Custom Content Control Insert Dialog

The first link uses text content controls built into Word. If you need a drop-down look at the second link and then I would suggest using one of the tools in the last two links. Both easily insert mapped drop-down controls.