In short: you can't any more, because the feature is no longer available. According to Chris Webb:
The cube browser has now disappeared from SSDT, and is replaced by the
control used by the SSRS query generator. This is because the old
browser used the OWC control which was deprecated years ago and
finally had to be replaced; it’s a massive step backwards because this
control is truly awful: for example you can no longer put anything
other than Measures on columns in your query.
You can confirm this by changing to design mode, and creating a simple MDX query with a dimension on the Columns axis:
SELECT
[Geography].[Country].[Country] ON COLUMNS
FROM [Adventure Works]
If I run this query in the SSAS browser, I get the error
Query preparation failed: ...The first axis [should not] reference any
dimension other than the Measures dimension"