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The Square integration on Make does not have a Search Customers module, which is what you would normally use to check if an email exists. This Search Customers endpoint already exists on Square.
I’m sure others would agree that this is a nice feature to have.
You can submit this suggestion to the Idea exchange, under App improvement ideas.
Don’t forget to search for it first, just in case someone already suggested it, so that you don’t end up creating a duplicate.
If the external service has a Developer API Reference/Documentation then you should be able to integrate the endpoints in Make using the app’s universal module (Make an API call) or generic HTTP “Make a request” module, something like this:
Then, to create a customer only when there are no results, take a look at this example:
That’s what the error directive Resume allows you to do. First, you use a Search module to see if it exists, then Create a new item if it doesn’t exist. In the Create field, you use an if-statement to “ignore” a required field if there are search results, throwing an error and allowing you to “skip” the creation module.
The if (not-exist-create-ignore) throws the error (ignore if exists), which the Resume directive can be used to return the item from the Search module (to check if it exists).
I explained this in a similar older post Can I skip a module based on a filter? - #3 by samliew