Check if WHMCS site has sertain customfield value from 3rd party

[JosephXXI]

Hello everyone,

I have a question, answer for which I can't by myself, so I need your help.

Background:
I have an admin only custom field. 
This field represents an API key for a service (the service) which is located on a separate server.

The key value will be stored only in the WHMCS site/server (the site) MySQL database.
Thit means that when somebody access the service it must validate provided API key by "asking" the site if the key is correct.

Question:
Is there a way to somehow pass an API key value to the site and get a bool result (True if the key exists in the DB, False otherwise)?
I need the check to be performed on the site (so no API keys info  info isspeaded outside the site).

Possible soulutions:
I was thinking about some way to execute a PHP function from the service but couldn't find any info about such approach.
Maybe there is a way to create a custom WHMCS API method?

P. S.
Ther is always an option to access the site's MySQL database directly, but that would give possibility for the service to read all API keys which doesn't sound secure...