Tinting poly is tricky but possible.The best way to do it is with tints , not stains. Adding stain to your poly can easily create a delamination problem because the carriers in the stain compromise the urethane.I use transtint dyes which can be bought or ordered through
www.woodcraft.com.
You can add a lot of brown color to a urethane this way without compromising the urethane . Also, tinting is not easy as too heavy of a tint can make the finish appear streaky.I sometimes tint the poly heavily but still apply a fairly thin coats .And usually, multiple coats are needed to get the right effect. I rarely would try to get a lot of color out of just one coat.