Is it possible to force an adaptive join with a query hint?
As it says in the documentation, adaptive join currently supports SELECT statements only:


  Adaptive join eligible statements
  
  A few conditions make a logical join eligible for a batch mode Adaptive Join:
  
  
  The database compatibility level is 140.
  The query is a SELECT statement (data modification statements are currently ineligible).
  The join is eligible to be executed both by an indexed Nested Loops Join or a Hash Join physical algorithm.
  The Hash Join uses batch mode - either through the presence of a Columnstore index in the query overall or a Columnstore indexed table being referenced directly by the join.
  The generated alternative solutions of the Nested Loops Join and Hash Join should have the same first child (outer reference).