Since my owner has lender how and where do i go about sending 20 day preliminary notice to lender just in case of nonpayment or delay in payments.

I have two current new construction homes here in California with BBVA as the lender and want to make sure that if they dont pay i can file mechanics lien later? Do I need to send 20 day prelim notice to lender and if so the local branch or headquarters and when do I need to do this before 100% completion? Thanks first time as a general contractor after 52 years in the business I feel this is necessary as owners arent paying on time or correct draw amounts when requested.