How do I add my husband’s IPhone to the list of “Find my Devices”on my phone. Thank you

Please refer to Share locations with family members and locate their lost devices on iPhone, which states "With Family Sharing, you can share your location with members of your Family Sharing group and help them find lost devices. When the family organizer sets up location sharing in Family Sharing settings, the organizer’s location is automatically shared with everyone in the family, including any new members added later. Then, family members can choose whether or not to share their location."

Refer to the document: Find people and share your location with Find My - Apple Support