For on the web or Excel, we only could use Microsoft templates, cannot use custom templates.
Per my research, organization assets library (organization templates) is not available in Word on the web or Excel on the web. It only supports PowerPoint on the web and desktop clients.
To get this option in the office web, I recommend you give suggestion in the feedback portal. Many features of current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding and support.
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