I found resolution:

command.CommandText = @"select * from ""TESTENTITY"" where substring(""NAME"" from cast(@p0 as Integer) for 4) = 'Item'";
command.Parameters.Add(new FbParameter("p0", 1));

You need to cast parameter @p0 to Integer.

P.s.

If the parameter @p0 can be 0, then you need to do a check for NULL:

command.CommandText = @"select * from ""TESTENTITY"" where substring(""NAME"" from cast(coalesce(@p0+1, 1) as Integer) for 4) = 'Item'";
command.Parameters.Add(new FbParameter("p0", 0));