Can you associate a Lead to an already existing Opportunity from groovy scripts or rest apis?

Summary:

I'm trying to make an association from Lead to Opportunity from an automated process without actually creating the Opportunity but referring to an already existing one. I can associate the lead to opportunity from within the UI but if I try to make the association from any other means I get an error.

Content (please ensure you mask any confidential information):

RestAPI trying to POST create the records with the intention of creating a webservice to make the association

Code Snippet (add any code snippets that support your topic, if applicable):

Groovy code to attempt to create a Lead→Opty relationship on the lead object.