Is this for a list or site workflow?

List workflow in SharePoint 365.

Fms.

If you use a site workflow then you can add a site workflow webpart to a page that should take care of this. I have a workflow that I want to start with a button too, and I plan to export my list workflow and create a site workflow then import what I've done with the list workflow.

Yes, but you and me need a javascript code to put into the button in the form to start a list or site WF.

Fms.

Correction to my statement above: You cannot import the list workflow template into a site workflow, so I will have to save it as a snippet to re-use the logic.

In the workflow settings check on start manually, check on enable workflow to start from the item menu, give a menu label or else it will take from workflow name, specify position if you want to order, on the form type select as custom, cross out everything from start page. After publishing this if you go to the list item and click on three ellipsis you will see the new label to start the workflow. 

This can now be achieved with the Component Workflow functionality - by enabling a Component Workflow to be started externally by HTTPS endpoint, you can wire up a button with some very simple JavaScript to post values to the workflow which will be started in a single request.