Q.when to prune crepe myrtle trees
I have my tree still in pot. Once planted, when shall I prune?

I would recommend early spring, while the tree is still dormant. Here are a couple of articles explaining more about pruning crepe myrtles as well as other aspects of their care:
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/best-crepe-myrtle-pruning-time-when-to-prune-crepe-myrtle.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/pruning-crepe-myrtle-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-care.htm