How to grant external providers access to a OnPremise Exchange account?

Hello,

I want to use cituro.com as a addition for my website to be able to provide easy online booking of service appointments. Therefore the provider needs to connect with my exchange account to be able to make new appointments within my calendar and to see when i am free to be booked. Here is described what would be required to do so:
https://www.cituro.com/help/kalender-synchronisierung/outlook-synchronisierung/
So far I only have a normal exchange account from OVH and I don't know if and with which option I would be able to extend my account to grant the website access to my calendar. If i try to link my account there, I always end at a Microsoft login where I am not able to use and link my OVH exchange account since this account seems to be unknown for the Microsoft servers...

I would appreciate very much, if someone can tell me if and how I could solve this.

Big thanks in advance and best regards,
Andi