If I miss the first rak'ah of a prayer and then complete it after the imam's taslim, do I recite the tashahhud or is it sufficient for me to say the taslim only after rising from the last prostration?

The final sitting (qa'dah akhira) and the tashahhud within it before the taslim are two obligatory acts (fard) without which the prayer is not valid. This is based on the statement of Ibn Mas'ud: "Before the tashahhud was made obligatory upon us, we used to say with the Messenger of Allah, peace and blessings be upon him: 'Peace be upon Allah before His servants... Peace be upon Jibril and Mika'il... Peace be upon so-and-so.' Then the Prophet, peace and blessings be upon him, said: 'Do not say: "Peace be upon Allah," for Allah is As-Salam. Rather, say: "At-tahiyyatu lillah...'" (Narrated by An-Nasa'i). This indicates its obligatoriness after it was not so, and this is the madhhab of Ash-Shafi'i, Ahmad, and Ishaq. It is necessary to perform the tashahhud before the taslim. As for performing the tashahhud with the Imam, it is for following only and is not the place for the worshipper's tashahhud if the prayer is four rak'ahs or two rak'ahs. However, in the Maghrib prayer, it is the middle tashahhud.