At the moment, there is no provision for this. Perhaps you could use the OnClick event of the Add Command Button to invoke a JS interop call that will find and .focus() the first input it finds on the grid, but this can become difficult if you have more grids rendered.
There is already a feature request for defining a default focus in the grid and I suggest you Follow it to get status updates (I have added your Vote for your): https://feedback.telerik.com/blazor/1408647-specifying-which-field-should-have-focus-in-grid-editor.
If you have suggestions on how this should be exposed for configuration, I encourage you to post them there.
UI for Blazor