I didn’t expect that I have to do literature search for this. But I did search a bit.
The “Fechtbuch” by Hans Talhoffer (15th century book) specify how to fight against mounted crossbowmen on horseback.
Die Illenden.
The pursuer
Wie sich ainer halten sol in der flucht mit dem armprost.
Just so should one behave in the flight with the crossbow.
Der wyl den under das pfert Renen.
This one then means to run beneath the horse.
Hie hatt der den geschossen und stoszt Im den Spiesz mit dem Armbrost hinweg und würt In ergryffen by dem halsz.
Here this one discharges, and then thrusts the lance at him, with the crossbow away, and grasps him about the throat
Hie wyl der den Recht empfahen mit dem Spiesz.
Here this one means to receive the one on the right with the lance.
Wie sich ainer halten sol mit dem Spiesz gegen ainem mit ainem Armbrost.
Just so should one with the lance behave against one with a crossbow.
From the book, mounted crossbowmen would charge and shoot at heavy cavalry. Shoot while at the side of heavy cavalry. Hit and run heavy cavalry.
There is a modern literature of ‘The Horse of Premodern European Culture’
For field battle, mounted crossbowmen were placed on the flanks of lancers and light cavalry.
Pierto Monte mentioned in his work ‘Collectanea’ that mounted crossbowmen are useful against German or Swiss pike block if bombard is absent.
Mounted xbowmen were used together with knight and loght cav to fight pikes/ heavy cavalry charge.