By default, Integromat connects to ArcGIS Online, but Integromat can connect with ArcGIS Enterprise instances through Portal for ArcGIS, as long as Portal for ArcGIS is publicly accessible, and Integromat is added as an application item. Refer to Survey123 Tricks of the Trade: Integromat for more information.
However, a change to the backend login process of Portal for ArcGIS has affected how Integromat authenticates and validates users, and connections from Integromat may exhibit some unexplainable behavior. In most cases, the error message: " grant_type invalid" is seen when making a new connection in Integromat to the portal, but also through the browser's developer tools when monitoring web traffic. There have been occurrences where Integromat connects to Portal for ArcGIS 10.5.1 successfully at first, but the connection fails upon further investigation.
Therefore, Esri recommends using or upgrading any current portal to Portal for ArcGIS 10.6.1 and above to successfully connect to Integromat.