You can do this using a repeated assignment and the reserved keyword 'time'. E.g.:
- Add a species 'drug_release' to your model
- Add the necessary parameters ym, yo, k to your model
- Add a repeated assignment to your model where the rule reads: 'drug_release = ym-(ym-yo)*exp(-k*time)'
Make sure your time units are consistent between the 'time' in your repeated assignment and the rest of your model.
Let me know if that works for you.