lookup - check if many result but nothing selected in javascript

Hi!

My problem:

I have a lookup on my form. If I write text in the textbox and multiple results were returned and I press 'Tab' I get an orange exclamation mark which prevents me from saving.

My problem here is, that I have to call a Service first before saving the form. If Service-Call is successfull I can save, otherwise not. But the service should not be called if saving the form is prevented. So how do I check that the lookup has more than one result. In other words: how to check if saving is possible?

thx

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