Try setting it with Processing Rules in your report suite. https://experienceleague.adobe.com/docs/analytics/admin/admin-tools/manage-report-suites/edit-report.... In the rule's condition, check if "c.a.activityMap.link" is set. If so, then under the rule's action, overwrite your eVar with that "c.a.activityMap.link". AA should then set your eVar with the Activity Map Link Name when it processes the hit.
You can also try using Dynamic Variables https://experienceleague.adobe.com/docs/analytics/implementation/vars/page-vars/dynamic-variables.ht.... Set your eVar with the value "D=activityMap.link". That should cause AA to copy the Activity Map's Link Name to your eVar when it processes the hit.
Note: while I've used Processing Rules and Dynamic Variables, I have not tried any of the above before with Activity Map Link Name. Please test the suggested methods properly on your own.
