Hello,
You can link the button to the action by using the action property of the button element. Here you can reference the defined table action.
Example:
<table id="tableId" dataContainer="tableDc">
<actions>
<action id="excelExport" type="excelExport"/>
</actions>
<columns>
<column id="myCol"/>
</columns>
<buttonsPanel id="buttonsPanel" alwaysVisible="true">
<button id="excelExportBtn" action="tableId.excelExport" />
</buttonsPanel>
</table>
For general examples have a look at the github projects of the devs. For example the demo petclinic project: https://github.com/Haulmont/jmix-petclinic-online-demo
Hope that helps.