Set signature placeholder in existing pdf file

I have pdf file. Is there any way to set signature placeholder using docusign in that pdf file?

I am not sure what's wrong here. so I am adding fully code here. It is not allowing me to save so adding some more text.

public static EnvelopeDefinition MakeEnvelope(string signerEmail, string signerName, string signerClientId, string docPdf)
{
    byte[] buffer = System.IO.File.ReadAllBytes(docPdf);

    EnvelopeDefinition envelopeDefinition = new EnvelopeDefinition();
    envelopeDefinition.EmailSubject = "Please sign this document";
    Document doc1 = new Document();

    string doc1b64 = Convert.ToBase64String(buffer);

    doc1.DocumentBase64 = doc1b64;
    doc1.Name = "Lorem Ipsum"; // can be different from actual file name
    doc1.FileExtension = "pdf";
    doc1.DocumentId = "3";

    envelopeDefinition.Documents = new List<Document> { doc1 };

    Signer signer1 = new Signer
    {
        Email = signerEmail,
        Name = signerName,
        ClientUserId = signerClientId,
        RecipientId = "1",
    };

    SignHere signHere1 = new SignHere
    {
        AnchorString = "/sn1/",
        AnchorUnits = "pixels",
        AnchorXOffset = "10",
        AnchorYOffset = "20",
    };

    Tabs signer1Tabs = new Tabs
    {
        SignHereTabs = new List<SignHere> { signHere1 },
    };
    signer1.Tabs = signer1Tabs;

    Recipients recipients = new Recipients
    {
        Signers = new List<Signer> { signer1 },
    };
    envelopeDefinition.Recipients = recipients;

    envelopeDefinition.Status = "sent";

    return envelopeDefinition;
}

Yes, sure, of course.
Here is an article showing how to do that and here is the relevant C# code:

https://developers.docusign.com/docs/esign-rest-api/how-to/request-signature-email-remote/

EnvelopeDefinition env = new EnvelopeDefinition();
env.EmailSubject = "Please sign this document set";
// Create document objects, one per document
Document doc1 = new Document();
string b64 = Convert.ToBase64String(Document1(signerEmail, signerName, ccEmail, ccName));
doc1.DocumentBase64 = b64;
doc1.Name = "Order acknowledgement"; // can be different from actual file name
doc1.FileExtension = "html"; // Source data format. Signed docs are always pdf.
doc1.DocumentId = "1"; // a label used to reference the doc
Document doc2 = new Document
{
DocumentBase64 = doc2DocxBytes,
Name = "Battle Plan", // can be different from actual file name
FileExtension = "docx",
DocumentId = "2",
};
Document doc3 = new Document
{
DocumentBase64 = doc3PdfBytes,
Name = "Lorem Ipsum", // can be different from actual file name
FileExtension = "pdf",
DocumentId = "3",
};
// The order in the docs array determines the order in the envelope
env.Documents = new List<Document> { doc1, doc2, doc3 };
// create a signer recipient to sign the document, identified by name and email
// We're setting the parameters via the object creation
Signer signer1 = new Signer
{
Email = signerEmail,
Name = signerName,
RecipientId = "1",
RoutingOrder = "1",
};
// routingOrder (lower means earlier) determines the order of deliveries
// to the recipients. Parallel routing order is supported by using the
// same integer as the order for two or more recipients.
// create a cc recipient to receive a copy of the documents, identified by name and email
// We're setting the parameters via setters
CarbonCopy cc1 = new CarbonCopy
{
Email = ccEmail,
Name = ccName,
RecipientId = "2",
RoutingOrder = "2",
};
// Create signHere fields (also known as tabs) on the documents,
// We're using anchor (autoPlace) positioning
//
// The DocuSign platform searches throughout your envelope's
// documents for matching anchor strings. So the
// signHere2 tab will be used in both document 2 and 3 since they
// use the same anchor string for their "signer 1" tabs.
SignHere signHere1 = new SignHere
{
AnchorString = "**signature_1**",
AnchorUnits = "pixels",
AnchorYOffset = "10",
AnchorXOffset = "20",
};
SignHere signHere2 = new SignHere
{
AnchorString = "/sn1/",
AnchorUnits = "pixels",
AnchorYOffset = "10",
AnchorXOffset = "20",
};
// Tabs are set per recipient / signer
Tabs signer1Tabs = new Tabs
{
SignHereTabs = new List<SignHere> { signHere1, signHere2 },
};
signer1.Tabs = signer1Tabs;
// Add the recipients to the envelope object
Recipients recipients = new Recipients
{
Signers = new List<Signer> { signer1 },
CarbonCopies = new List<CarbonCopy> { cc1 },
};
env.Recipients = recipients;
// Request that the envelope be sent by setting |status| to "sent".
// To request that the envelope be created as a draft, set to "created"
env.Status = envStatus;
return env;