You have to find the form and call the method, e.g.
using System.Linq;
...
Application.OpenForms
.OfType<Form1>() // Among the all opened forms of Form1 type
.LastOrDefault() // Take the last one (or null if there's no such form)
?.AfterConnect(); // And call AfterConnect() on it (if the form has been found)
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solved C#, Winform: How to pass information from one form to another [duplicate]