Hi there,
Within your signup form, you’ll want to ensure that you’re using a date input to collect birthdays. Once you do that, you’ll have the option to map the date that customers enter, to a custom profile property. This property should not exist by default so you’ll want to create your own property and label it Birthdate. From there, anybody who signs up from your form and enters their birthday should have the Birthdate custom profile property.
This new Birthdate profile property should be the one that triggers your birthday flow.
I hope that helps!