Write simple code for ACF implementation on WP site (Advanced Custom Fields on WordPress...external link (when entered with ACF) applied to featured image.

OK, so as the title says... I need someone to write me a simple function (or series, I don't know much about PHP which is why I'm here) to use the ACF field I have set up (external_link is the slug) wrap around the featured image, and if no "External Link" is added, then it just acts as normal. This should be pretty straight forward and simple for anyone familiar with ACF and/or WordPress development, I just really struggle with it and don't have the time today to learn what needs to be done. If you take this job, I need you to provide me with the bit of code, tell me where to place it (if not in [login to view URL] of child theme), and briefly explain to me what's happening in the code (I can read most of it, but sometimes I struggle...I just like to know what's happening and where for future reference). Thanks for taking the time to read this! ~~~FYI~~~ I do not intend to spend much on this at all... I hope you understand what I mean, and I hope you don't waste either of our times with bids in the double digits :D Have a great day!