The Experience Cloud ID is found in the data collection query string, and uses the parameter "mid". A list of the data collection query parameters can be found at https://docs.adobe.com/content/help/en/analytics/implementation/javascript-implementation/data-collection/query-parameters.html . You can use the dynamic variable approach in Adobe Launch, which is outlined at https://docs.adobe.com/content/help/en/analytics/implementation/javascript-implementation/variables-analytics-reporting/dynvars...

As you mentioned that mid is available in querystring parameter, you can fetch the mid from querystring using function s.Util.getQueryParam and assign to evar.Util.getQueryParam Other method as suggested by Andrey, you can use eVar1="D=mid"; to pull the mid and assign it directly from analytics server call mid parameter to evar.