Hi, i am having problem linking Account record with a custom module record.

I created a module named Therapist whose record will be linked to an Account automatically. 

In the module Account, i created a related list item Therapist as a multiplelookup field. I can update its record manually but not automatically.

I tried using API to update Account with passed it param containing Therapist ID but related list didn't change. Then, i tried linking API https://sandbox.zohoapis.com/crm/v2/Accounts/Account_id/Therapists/Therapist_Id but keeps getting error message "the relation name given seems to be invalid".

Are there any other APIs/solution can i use? Please help.

Thank you.