How to accept lead investor commitment on a schedule

Closing a pre-seed round 1.4 m USD for 18 month,

the lead investor wants to invest on a quarterly basis, instead of a full commitment upfront.

How would you structure the shareholder agreement would I still put the total amount raised and the no shares the investor would receive

+ a schdule for the inbestment to flow in

OR should I not do a shareholder agreement, since the investor wouldn't be receiving the shares upfront ?