You can mix it into the trace mix.
See here:
Practical PMDD Information
As you can see, EI/PMDD and the other incarnations go back well over a decade.
Few folks refer to PMDD much these days and even fewer used PMDD in the original form.
So a 1:1 ratio of MgSO4 should suffice with plantex.
So dose 2-3x a week etc like the traces.
You can likely dose once a week also after a water change.
Some bozos claim dosing it carefully daily helps their plants.
I've never seen this(I tried that) and I have little trouble growing any plant dosing Mg once a week........so that's the control that shows it's not really doing much and that some other thing is giving their results...........
You repeat it a few times, then it's pretty plain.
Regards,
Tom Barr