You need to use the __Sortorder field to sort on. That is y default not in your index though so you need to add that as well. The explanation can be found in detail on this blog: https://www.sitecoreinsiders.com/sxa-search-results-sorting-by-sortorder-field/
In case that disappears, this is the short version:
This field is being excluded from the index by ...
I don't think SXA provides this OOTB, we need to create QueryToken for Multlist With Search. The Multilist with Search field uses a SourceFilterBuilder class to parse its field source. Sitecore uses SourceFilterBuilderFactory to create the SourceFilterBuilder
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Needs to ...