The below code snippet shows annotating the plan element. Here in the code alignment element is annotated with the station values along an interval.

//Required References
using System;
using Bentley.DgnPlatformNET;
using Bentley.MstnPlatformNET;
using Bentley.CifNET.GeometryModel.SDK;
using Bentley.GeometryNET;
using Bentley.DgnPlatformNET.Elements;
using System.Diagnostics;
using Bentley.CifNET.LinearGeometry;

public void AnnotatePlan()
        {

            try
            {
                //Get active dgn model
                DgnModel model = Session.Instance.GetActiveDgnModel();

                //Get active geometric model to get alignments
                Bentley.CifNET.SDK.Edit.ConsensusConnectionEdit con = Bentley.CifNET.SDK.Edit.ConsensusConnectionEdit.GetActive();
                GeometricModel geomModel = con.GetActiveGeometricModel();

                //Get all alignments from current DGN 
                foreach (Alignment alignment in geomModel.Alignments)
                {
                    if (!alignment.IsFinalElement)
                        continue;

                    //Get the alignment points at defined length and offset for placing the annotation
                    if (alignment != null)
                    {
                        LinearElement LinearEl = alignment.LinearGeometry;
                        double EndPosition = LinearEl.Length;
                        double Interval = 100;
                        double Length = 20;
                        for (double dStation = 0; EndPosition > dStation - Interval; dStation += Interval)
                        {
                            if (dStation > EndPosition)
                                dStation = EndPosition;
                            LinearPoint offsetPt0 = null, offsetPtLength = null;
                            offsetPt0 = LinearEl.GetPointAtDistanceOffset(dStation, 0);
                            offsetPtLength = LinearEl.GetPointAtDistanceOffset(dStation, -Length);

                            //User need unit conversion here based on unit settings 
                            DPoint3d startPoint = offsetPt0.Coordinates;
                            
                            //User need unit conversion here based on unit settings 
                            DPoint3d endPoint = offsetPtLength.Coordinates;
                          
                            DSegment3d segment = new DSegment3d(startPoint, endPoint);
                            LineElement lineElement = new Bentley.DgnPlatformNET.Elements.LineElement(model, null, segment);
                            lineElement.AddToModel();

                            DVector3d rotVector = segment.UnitTangent;
                            DMatrix3d rotMatrix = DMatrix3d.Rotation(2, rotVector.AngleXY);

                            //Create new DgnTextStyle element for annotation definition
                            DgnFile activeDgnFile = Bentley.MstnPlatformNET.Session.Instance.GetActiveDgnFile();
                            DgnTextStyle textStyle = DgnTextStyle.GetByName("Line Length Label", activeDgnFile);

                            if (null == textStyle)
                            {
                                textStyle = new DgnTextStyle("Line Length Label", activeDgnFile);
                                textStyle.SetProperty(TextStyleProperty.Width, 400D);
                                textStyle.SetProperty(TextStyleProperty.Height, 400D);
                                textStyle.Add(activeDgnFile);
                            }

                            //User might need unit conversion for dStation based on unit settings 
                            string textString = string.Format("sta = {0:F1}", dStation);

                            //New TextBlock for defining annotation
                            TextBlock textBlock = new TextBlock(textStyle, model);
                            textBlock.AppendText(textString);
                            textBlock.SetUserOrigin(startPoint);
                            textBlock.SetOrientation(rotMatrix);

                            //Add annotation
                            TextElement textElement = (TextElement)TextElement.CreateElement(null, textBlock);
                            textElement.AddToModel();
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                Trace.WriteLine(ex.Message);
            }
            return;
        }

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Output

The above code annotates alignments as shown in below image