There are only two options are available. 1. space and 2. comma.
If you want to set comma as delimiter for tag than go to
Admin > Posting and there find option
Use comma as the only tag separator and select it.
If you want to change the custom behaviour for this then look for the following file:
You have to modify two functions - qa_tag_click & qa_tag_typed_parts in that file.
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July 15, 2020: Q2A 1.8.5 released!