Visa Expiry date

I am applying for dependent partner visa to join my spouse who is already in the UK with student route. In my IHS page, can I enter the "Leave to Enter" residence permit validity date as the visa expiry date? Or should I enter the course end date as the visa expiry date?

Just whatever is the end date of their Tier 4 or Student permission. If they don't know, they can look at their Biometric Residence Permit.