Hello ola-ola,
First off, this article explains more about setting up the Apple Watch for a family member: Set up Apple Watch for a family member. It explains that you need: "A Family Sharing group that includes the person who'll use the Apple Watch. You need to have the role of organizer or parent/guardian to set up an Apple Watch for a family member."
The person who sets this watch up will need to already be designated as the Family Organizer or the Parent. When the organizer adds an adult to the Family Sharing group, they can designate them as a parent or guardian. It can't be done later. You might be able to have them remove the person from the Family Sharing group, then add them back with the new role assigned at that time. This is explained more here: Manage Family Sharing settings.
Let us know if we can help with anything else at all. Thanks and have a fabulous day!