Set imageClickMode="OPEN_URL" (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section) and set a linkURL for your main image. In JuiceboxBuilder-Pro, just click the corresponding thumbnail on the 'Images' tab and you'll see the place to enter your LinK URL at the bottom of the window.

When imageClickMode="OPEN_URL", normally Juicebox-Pro will open the imageURL (the image itself) in a new tab but if the image has a linkURL assigned to it, then the linkURL will be opened instead.

If you manually edit your gallery's 'config.xml' file, then the <image> entry might look something like this:

<image imageURL="images/image_0001.jpg"
    thumbURL="thumbs/image_0001.jpg"
    linkURL="images/image_0001.jpg"
    linkTarget="https://www.google.com/maps?q=37.819722,-122.478611">
    <title><![CDATA[Image title text goes here.]]></title>
    <caption><![CDATA[Image title text goes here.]]></caption>
</image>

Otherwise, you can add a link to an image title or caption using HTML formatting (or more specifically an HTML <a> tag) as noted in this FAQ:
How do I add HTML formatting to image captions and titles? - https://www.juicebox.net/support/faq/#custom-3

You could enter HTML such as the following into JuiceboxBuilder-Pro's interface:

<a href="https://www.google.com/maps?q=37.819722,-122.478611">Click here to view in Google Maps.</a>

If you manually edit your gallery's 'config.xml' file, then you'll need to wrap the HTML inside <![CDATA[ ... ]]> tags and the <title> (or <caption>) entry might look something like this:

<title><![CDATA[<a href="https://www.google.com/maps?q=37.819722,-122.478611">Click here to view in Google Maps.</a>]]></title>

I hope this helps.