You can use InsertService
in that case:
local InsertService = game:GetService("InsertService")
local ShirtID = 5025324849 -- ID from catalog
local model = InsertService:LoadAsset(ShirtID) -- it inserts it as a model
model.Shirt.Parent = Character -- Or just get the model.Shirt.ShirtTemplate and set the Character's ShirtTemplate to that
More on InsertService: InsertService | Documentation - Roblox Creator Hub