OptinMonster is a plugin that enables you to create a variety of forms to engage customers. Email addresses and subscriber information can be sent directly into Klaviyo via the OptinMonster-Klaviyo integration.
While Klaviyo does not have a built-in integration with OptinMonster, you can still integrate your OptinMonster account with Klaviyo. This is possible via an integration available though OptinMonster.
You'll find integration instructions on OptinMonster's website: How to Connect OptinMonster with Klaviyo.