HitCal

Hitcal is a keyboard app you can use with any app in which you can see a keyboard in your iPhone. It will present you the calendar where you can view your free time slots. Choose the free time slots you can offer to the other party and a link is inserted into your message or email.

When the other party receives your message or email and select a time slot that is suitable to them, the meeting is scheduled and entered in both of your calendars.

Source credit: https://www.hitcal.com/