Question from Reddit user:
I have a Facebook Pixel with several custom conversion events, including purchase completion.
That’s all been working fine with this new client since the start of March. It’s set up with a purchase completion value of £1 as a default as the platform my client is using does not allow me to inject Facebook conversion code so I have to rely on the URL only to track conversions. (So I can’t put the sale value into the event.)
I am now attempting to create a lookalike audience based on past customers who have converted to sale but Ads Manager, in step 2 “Select an event with value” is not allowing me to select my custom conversion event as an “other event with value”.
It’s in the list of events but is greyed out as FB seems to think there is no value associated with the event. I just don’t understand why it cannot see the default value I’ve associated with the event from the outset.
Any ideas on how to fix this? Could it just be the audience size isn’t big enough?… it’s under 1000 people.
Answer from Nabil:
The short answer is:
The reason your custom conversion event is greyed out in the “Select an event with value” step is likely that the audience size is indeed too small; Meta typically requires a source audience of at least 1,000 active users to generate a Lookalike Audience, and often more to be effective.
However, the core technical issue is that Facebook’s system is very strict about seeing the value parameter passed with the event data.
Since you are relying only on a URL-based conversion and using a default value in the setup, Facebook’s algorithm might be reading the event without the dynamically sent value parameter in the pixel code, even if you set a default.
The definitive solution to ensure the value is always seen and to future-proof your tracking is to implement the Facebook Conversions API (CAPI).
The long answer is:
It’s understandable that you’re running into this frustration, as Facebook’s (Meta’s) systems can be finicky about custom audiences.
While the under 1,000 people count is the most probable immediate cause for the greyed-out option, the underlying problem is how the value parameter is being transmitted.
Facebook’s Lookalike Audience creation process, particularly for events with value, requires confidence that the conversion events are receiving a structured value parameter.
When you rely solely on a URL-based event trigger, your client’s platform is not executing the necessary Facebook Pixel code with a dynamic value and currency parameter.
Even if you set a default value of £1 in the Facebook interface, the platform may not recognize this as a transactional event for Lookalike purposes, as it prefers to see the event fire with an explicit value parameter attached to the purchase event code.
To permanently fix this and also solve the missing sale value issue, the most powerful and scalable solution is to implement the Facebook Conversions API (CAPI) using a server-side solution like Stape or Google Cloud Platform (GCP), managed via Google Tag Manager (GTM).
With this setup, you use GTM’s client-side container to capture the form submission or checkout completion.
Instead of relying on the client’s browser to fire a pixel, GTM sends the conversion event data, including the full customer value, currency, and other parameters, directly to your server-side container.
The server-side container then sends this complete, reliable data payload to the Facebook Conversions API.
Because this is a server-to-server connection, it bypasses browser limitations, ad blockers, and cookie restrictions, ensuring Meta receives every conversion, along with the correct, dynamic sale value (which you can pull from your client’s backend or data layer), consistently.
This guarantees that your purchase event is properly registered with a value and will become eligible for Lookalike Audience creation as soon as your audience size grows above the 1,000-user threshold.
It also allows you to implement Advanced Matching to improve audience size and accuracy.
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