Here's how to use Windows 10 Paint 3D for IMVU.
First make the object, a simple chair made from some blocks or other simple shapes - note its position relative to the canvas, it may not be centred or grounded as might be expected (for IMVU use).
Once made, click "Menu" then "Save" to save the project locally. Click "Menu" again and select "Export File". To the right click the "3D FBX" button and in the dialogue that appears type a different file name as needed then click "Save".
With an FBX then available, open IMVU, access "Create" mode and start a new furniture project deriving from the default chair. In the Editor (that appears) click the "fbx import" tab, then the "Load FBX" button. Browse to and select the Paint 3D exported *.fbx and click "Open".
In the "Select Skeleton Root" pop-up make sure "RootNode..." is selected then click "Configure FBX". This loads the FBX data into the importer. Here change "Mesh IDs", "0" overrides the default mesh, "1", "2", "3" etcetera depending on the number of meshes.
Check "Materials" are shown and selected then change "Apply scale:" typing at least "1000", but more typically 1700, then click the "Import" button. IMVU will convert the file and display a blank panel. Once that happens click the yellow "Apply Changes" button to update the project with IMVU automatically assembling the imported components.
Depending on the product the final step is to click the "config" tab and select the original derived items skeleton ("furniture_lo_chair01.xsf"). "Apply Changes" again, save and upload, or edit clicking the "meshes" or other tab as needed.