If you are trying to add a custom caption to just one image using the Juicebox Plugin for Lightroom, then do the following:
(1) Click the 'Library' module tab.
(2) Select the image you want to add the caption to.
(3) In the 'Metadata' section, add the following to the 'Caption' box (changing the link and/or text as appropriate):
<a href="https://www.roatan.photos/credits.html">Click here to go to 'credits' page.</a>
(4) Click the 'Web' module tab.
(5) Select 'Juicebox-Pro' as the 'Layout Style'.
(6) Scroll down to the 'Image Info' section and make sure that the 'Caption' is set to {Caption»}. (This is the default value so you should not need to change anything.)
(7) Save your gallery to your hard drive via the 'Export...' button.
Step #3 embeds the caption as the IPTC Caption within the image.
Step #6 uses each image's IPTC Caption as the gallery's image caption. (Images without an IPTC caption will simply have a blank/empty caption.)
Alternatively, you could either:
(1) Open your gallery in JuiceboxBuilder-Pro (after creating it in Lightroom), click the appropriate thumbnail on the 'Images' tab and enter the caption (below) in the 'Caption' input box before re-saving the gallery on the 'Publish' tab.
<a href="https://www.roatan.photos/credits.html">Click here to go to 'credits' page.</a>
... or:
(2) Edit your gallery's 'config.xml' file in a plain text editor (again, after creating it in Lightroom) and replace the image's <caption> tag manually, e.g.:
<caption><![CDATA[<a href="https://www.roatan.photos/credits.html">Click here to go to 'credits' page.</a>]]></caption>
I hope my notes above help.