In addition to what @MANSIR2094, there may be another way depending on your use case. I’m not super clear why you want to subscribe them to your first List, and then subsequently add them to another List. But here’s how to do it within Klaviyo.
If a user is initially Subscribed to a List based on your API action when click on the button, you can create a Flow that simply adds them to another List all within Klaviyo so you don’t have to make a roundtrip API call to subscribe them to yet another List.
For example in your case, when they click on the button, you Subscribed them to a List (“Subscribers_Klaviyo Button” List). If you use the “Subscribed” API call, that will automatically mark the consent status (Action #1) and it will abide by whatever List settings you have for single or double-optin.
Then build a a List Triggered Flow off of that initial List subscription. The only purpose of this Flow is (your Action #2) to add them to your target List using the “List Update” action in that initial Flow. You can add additional Flow Filters or conditional branches as a sanity check before adding them to your target List.
You can read more about that here:
Does this help or make sense?
