AB Test Custom Conversion

If you are testing for whole emails. I would suggest try out different layout of text format or maybe different CTA. 

In most of the cases CTA gets more click than the header so your test will be biased on that. 

You can rethink of the another strategy maybe.

Marketo often:

• Normalizes URLs
• Groups links if only tracking params differ

Always make links visually and structurally distinct, not just analytically different. You must use unique URLs (not just UTMs) for reliable A/B winner tracking

Yes, you are 100% correct on the workflow. Because you want to pick the winner based only on the specific header image clicks (and ignore other links in the email), you must use the Custom Conversion route.

Adobe Marketo Engage’s standard "Clicks Link" winning criteria in A/B tests does not distinguish links that differ only by query string parameters like UTM tags. Marketo treats URLs with identical base paths as the same link, which prevents accurate winner tracking based on those parameters alone.

Per my understanding to work around this limitation and ensure Marketo tracks the links as distinct in A/B tests, you have two effective options:

Option 1: Use a URL Anchor / Hash Fragment 

Append a hash or anchor to the end of the URLs (after any UTM parameters). For example:

• https://example.com/page?utm_term=A#A

• https://example.com/page?utm_term=B#B

Option 2: Use Two Different Redirect or Landing Pages

Create or use two slightly different URLs (such as vanity URLs or distinct redirects) that both ultimately lead to the same final destination. For example:

• https://example.com/pageA

• https://example.com/pageB

Additional Note:

• Test your links to ensure clicks are tracked correctly in Marketo.
• Use the Marketo Engage reporting interface to verify that clicks on each link are recorded separately.

Hope that helps?