In case of a new command that is added through Command Designer, we can specify an existing Icon using the Use Icon option.
We can also search and specify an existing Web Resource
Similarly, we can use Icon Gallery (XrmToolBox Plugin) to search for the existing web resource image and icons.
We can then use Copy Name option to get the full path of the web resource
msdyncrm_/KnowledgeManagement/_imgs/KnowledgeSearchProvider.svg
use it to specify Modern Image for our buttons
Hope it helps..
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