stratego | repository contains the Spoofax implementations

As mentioned by others in the comments, minimax should be able to notice if there is a danger in delaying to capture a piece automatically, and changing the evaluation function to force it to prefer earlier captures is likely to be detrimental to playing performance.

Still, if you really want to do it, I think the only way would be to start storing extra information in your game states (not only the board). You'll want to store timestamps in memory for every game state which allow you to still tell in hindsight exactly at what time (in which turn) a piece was previously captured. Using that information you could implement a decay factor in the evaluation function used in leaf nodes of the search tree.

A different solution may be to simply make sure that you search to an even depth level; 2 or 4 instead of 3. That way, your algorithm will always evaluate game states where the opponent had the last move, instead of your computer player. All evaluations will become more pessimistic, and this may encourage your agent to prefer earlier rewards in some situations.

This effect where odd search depths typically result in different evaluations from even search depths is referred to as the odd-even effect. You may be interested in looking into that more (though it's typically discussed for different reasons than what your question is about).