Currently, editing the source code of your Wix site isn't possible.
If you'd like to insert external HTML code you can do so using the HTML app.
With the HTML App you can add your own HTML code or embed another site onto your Wix site.
To add the HTML App: From the left side of the Editor in WIX, click the Add icon > clickApps > click HTML.
Click the HTML App to select it > from the pop-up menu, click Settings > underMode, choose HTML Code or Website address > enter the code/address > clickUpdate.