How do I add a tint color to a certain part of a texture?

I have a texture for my character with a white t-shirt.

How do I tint only a certain part of my texture, say the rectangle from (0,0.3) to (0.5,1)?
I am not proficient with shader programming.

Where exactly should I add a line of code, and which line of code exactly?
I want to multiply the texture’s color by my tint color.

In general, as a good start point to understand Surface Shader writing in Unity, I always recommend the examples : Unity - Manual: Surface Shader examples

And to do strictly what you said, try this shader :

Shader "Example/Diffuse Texture" {
    Properties {
      _MainTex ("Texture", 2D) = "white" {}
      _MyColor("My Color", Color = (1,1,1,1)
      _MyRectangle("My Rectangle", Vector) = (0, 0.3, 0.5, 1)
    SubShader {
      Tags { "RenderType" = "Opaque" }
      #pragma surface surf Lambert
      struct Input {
          float2 uv_MainTex;
      sampler2D _MainTex;
	  float4 _MyColor;
	  float4 _MyRectangle;
      void surf (Input IN, inout SurfaceOutput o) {
		  float4 c = tex2D (_MainTex, IN.uv_MainTex).rgb;
		  if (
			IN.uv_MainTex.x > _MyRectangle.x &&
			IN.uv_MainTex.x < _MyRectangle.z &&
			IN.uv_MainTex.y > _MyRectangle.y &&
			IN.uv_MainTex.y < _MyRectangle.w )
			  c.rgb *= _MyColor.rgb;
          o.Albedo = c; 
    Fallback "Diffuse"