Found info here, works for me:
It's an old topic but I have a trick so that you can add Auto-complete
function for fluid, vhs, flux.
First of all, download the XSD schema files from here :
https://fluidtypo3.org/viewhelpers/fluid/master.html put them
somewhere locally.
Next step, in Netbeans, go to menu Tools > DTD and XML Schemas, add
your 3 DTD in user catalog :
Flux : Public ID = http://typo3.org/ns/FluidTYPO3/Flux/ViewHelpers URI
= path to your flux XDS file
Fluid : Public ID = http://typo3.org/ns/TYPO3/Fluid/ViewHelpers URI =
path to your fluid XDS file
Vhs : Public ID = http://typo3.org/ns/FluidTYPO3/Vhs/ViewHelpers URI =
path to your flux VHS file
Next, in project properties, go to 'testing' menu and add the folder
where you stored your DTD.
Now, in your Project > Test Files, add a new XML Document and choose
"XML Schema-Constrained Document" on next step, click the button
'Browse' and By File > Your Project > Test Files and select import for
all the xsd schema files.
Then, change the prefix f for fluid, flux for flux and vhs for vhs
choose fluid as primary and finish.
A file is created with this content :
And into the section <f:alias>, you have the auto-completion for
fluid, vhs and flux.
<f:alias
        xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
        xmlns:f='http://typo3.org/ns/TYPO3/Fluid/ViewHelpers'
        xmlns:vhs='http://typo3.org/ns/FluidTYPO3/Vhs/ViewHelpers'
        xmlns:flux='http://typo3.org/ns/FluidTYPO3/Flux/ViewHelpers'
        xsi:schemaLocation='http://typo3.org/ns/TYPO3/Fluid/ViewHelpers file:/home/florian/Documents/Docs/Netbeans/Autocomplete%20Fluid/Schemas/Fluid.xsd
   http://typo3.org/ns/FluidTYPO3/Vhs/ViewHelpers file:/home/florian/Documents/Docs/Netbeans/Autocomplete%20Fluid/Schemas/Vhs.xsd
   http://typo3.org/ns/FluidTYPO3/Flux/ViewHelpers file:/home/florian/Documents/Docs/Netbeans/Autocomplete%20Fluid/Schemas/Flux.xsd'>
   
        Auto-completion here
       
</f:alias>
http://netbeans-org.1045718.n5.nabble.com/Schemas-for-code-completion-with-xsd-tt5752294.html#none
Some more info Use Netbeans to Create Sample XML from XSD
From a project that contains the .xsd:
- Click 
New File on the tool bar or File > New File from the menu 
- Choose 
XML > XML Document 
- Name your file, click next
 
- Choose 
XML Schema-Constrained Document, click next 
- Click the 
Browse button and use By File to navigate to the .xsd 
- Click the 
Import check box beside the file name, click next 
- Review the options available and click 
Finish to generate 
You can use more than one .xsd file to generate the .xml.  In this case it will use whichever .xsd file you choose as Primary to resolve any conflicts.