Install the wsp file as sandbox solution in your site, and prepare some webapplication in any server with IIS, in side or out side of your farm.
To more information visit here:
https://irunsharepoint.wordpress.com/installation-sharepoint-workflow-starter/
This is only for onpremise SharePoint 2013/2016 versions.
NOTE: .NET 4.5.2 is required to run the Web Services.
Last version include:
() SharePoint form pass parameters fix
() Auth webconfig fix
(*) Start Sites Workflows capability is added using the parameter itemId equal zero