If you want to use coordinates for -seed_sphere, I think that’s a bit tricky to achieve with MRtrix tools alone but can be done with a small C++ command:
First, make sure your coordinates are in scanner space of the T1w image (mrview → view tool. BTW you can copy these by pressing the copy button which also prints them to the terminal.)
To get these coordinates into the scanner space of the dMRI image, you could try out this command (untested):
It takes in a text file with scanner space coordinates (x,y,z per row) and a warp and outputs the warped locations. You’d need to use the inverse warp compared to what you’d use for transforming the T1w to dMRI space. If you don’t have a warp but only a linear transformation, see: linear transform track file - #2 by maxpietsch
Either copy the coordinatewarp.cpp file into your mrtrix3/cmd directory and run ./build or make an external module.