https://learn.microsoft.com/en-us/rest/api/power-bi/admin/capacities-assign-workspaces-to-capacity
Admin - Capacities AssignWorkspacesToCapacity - REST API (Power BI Power BI REST APIs) | Microsoft...
Assigns the specified workspaces to the specified Premium capacity. Permissions The user must be a Fabric administrator.
rest apipower biadmincapacitiesapis