Ref. No. 870/41-134
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
You can make niyyat of such optional prayers along with Salat al-Tasbeeh prayer. It is written in Fatawa Mahmoodiya that making niyyat of Istighfar, Salatul hajat etc in Tahiyyatul Wudu is allowable. The same is mentioned in other Fiqh books. (Fatawa Mahmudiya 7/712)
وکذا یصح لو نوی نافلتین او اکثر کمالونوی تحیۃ مسجد وسنۃ وضوء وضحی وکسوف (حاشیۃ الطحطاوی باب شروط الصلوۃ وارکانہا 1/216)
And Allah knows best
Darul Uloom Waqf Deoband
This answer was collected from the official website of Darul Uloom Deoband Waqf. The institution is based in India and the answers are provided in accordance to the Hanafi Fiqh.