Assuming you've worked with models previously that were drag and drop, they were probably utilizing modular avatar to automatically complete the parenting of the armatures.
What that would look like in unity without any tools would be opening the armature tab on the clothing, and then simply dragging each of the items under that to the base avatar armature ex.) Hips parented to Hips, Chest parented to Chest, etc. If for example an avatar had a layout with the armature similar to Upper_Arm.L/Ribbon, then you'd parent the Ribbon bone along with the Upper_Arm.L, etc.
This is the old, manual method and could easily be applied to correct this issue, but I believe the specific reason this is occurring for you is that you have 1.0 instead of 1.1 of the mamehinata dance wear.
I would suggest trying this version since there was an update in 10/9/2023 specifically to set up the outfit for modular avatar compatibility. Assuming there are no issues with your project, this outfit should truly be drag and drop, assuming also that you are attaching the outfit to the base avatar. The modular avatar component MA Mesh Settings will give a warning reading: "This component needs to placed inside your avatar to work properly." if the outfit is not parented to the avatar.
If the issue persists, feel free to post again and show the hierarchy and inspector for the outfit, it's possible something is being missed.
