Execute a JavaScript REST API call to validate metadata about the person in a person field

have a need to validate 2 people fields after the person is selected so that the form cannot be submitted if the selected people are not managers/leadership. We already have a rest services that ties into SAP to provide all of this information that returns a true or false JSON result. How can I integrate this into my form. other than tying it to the Submit button? Id like it to be validated when the control loses focus. 

This is the actual call to the service:

url: encodeURI(uri),
//not including URL for security, but the URL passes the user and the minimum level needed
type: "GET",
contentType: "application/json",
success: function (response) {
var isManagementOrLeadership = response.value;
Thanks for any help
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