Drawing Canvas

Paintable.cs

using System.Collections;
using System.IO;
using UnityEngine;
using UnityEngine.InputSystem;

public class SignatureCapture : MonoBehaviour
{
    [SerializeField] private GameObject brushPrefab;
    [SerializeField] private float brushSize = 0.1f;
    [SerializeField] private RenderTexture renderTexture;

    private Camera mainCamera;
    private InputAction drawAction;
    private InputAction touchPositionAction;

    // Initialization
    private void Awake()
    {
        mainCamera = Camera.main;

        // Set up the new Input System
        drawAction = new InputAction("Draw", InputActionType.Value, "<Pointer>/press");
        touchPositionAction = new InputAction("Position", InputActionType.Value, "<Pointer>/position");
        drawAction.Enable();
        touchPositionAction.Enable();
    }

    private void OnEnable()
    {
        if (drawAction != null)
        {
            drawAction.Enable();
        }
        if (touchPositionAction != null)
        {
            touchPositionAction.Enable();
        }
    }

    private void OnDisable()
    {
        if (drawAction != null)
        {
            drawAction.Disable();
        }
        if (touchPositionAction != null)
        {
            touchPositionAction.Disable();
        }
    }

    // Update is called once per frame
    private void Update()
    {
        DrawWithBrush();
    }

    // Draw with the brush when the left mouse button is pressed or touch input is detected
    private void DrawWithBrush()
    {
        if (drawAction.ReadValue<float>() > 0)
        {
            Vector2 screenPosition = touchPositionAction.ReadValue<Vector2>();
            Ray ray = mainCamera.ScreenPointToRay(screenPosition);
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit))
            {
                // Instantiate a brush
                GameObject brushInstance = Instantiate(brushPrefab, hit.point + Vector3.up * 0.1f, Quaternion.identity, transform);
                brushInstance.transform.localScale = Vector3.one * brushSize;
            }
        }
    }

    // Save the signature as an image
    public void Save()
    {
        StartCoroutine(SaveCoroutine());
    }

    private IEnumerator SaveCoroutine()
    {
        // Wait for rendering
        yield return new WaitForEndOfFrame();

        string filePath = Path.Combine(Application.dataPath, "savedSignature.png");
        Debug.Log($"Saving signature to {filePath}");

        // Set active texture
        RenderTexture.active = renderTexture;

        // Convert RenderTexture to Texture2D
        Texture2D texture2D = new Texture2D(renderTexture.width, renderTexture.height);
        texture2D.ReadPixels(new Rect(0, 0, renderTexture.width, renderTexture.height), 0, 0);
        texture2D.Apply();

        // Write data to file
        byte[] imageData = texture2D.EncodeToPNG();
        File.WriteAllBytes(filePath, imageData);

        // Reset active texture
        RenderTexture.active = null;
    }
}

this script at this time is untested but looks usable to me.