With SDL use SDL_TEXTINPUT and SDL_ttf to enter and display text.

sdl_textediting.cpp

#include <SDL.h>
#include <SDL_ttf.h>
#include <iostream>

int main(int argc, char* argv[])
{
	const std::string app_title = "SDL_TextBoxExample";
	const SDL_Color background_color = SDL_Color{ 0, 50, 255, 255 };
	const SDL_Color foreground_color = SDL_Color{ 255, 255, 255, 255 };
	const SDL_Rect window_rect = SDL_Rect{ SDL_WINDOWPOS_CENTERED , SDL_WINDOWPOS_CENTERED , 1280, 720 };
	const std::string font_path = "./assets/fonts/Roboto/Roboto-Bold.ttf";
	const int font_size = 32;
	SDL_Texture* text_texture = nullptr;
	SDL_Window* main_window = nullptr;
	SDL_Renderer* renderer = nullptr;
	TTF_Font* font = nullptr;
	bool should_quit = false;
	int app_return = 0; // 0 for success; 1 for failure

	try {
		std::cout << "Initializing SDL..." << std::endl;
		if (SDL_Init(SDL_INIT_EVERYTHING) != 0) {
			throw std::runtime_error(std::string("SDL_Init Failure: ") + SDL_GetError());
		}

		std::cout << "Initializing SDL_ttf..." << std::endl;
		if (TTF_Init() != 0) {
			throw std::runtime_error(std::string("TTF_Init Failure: ") + TTF_GetError());
		}

		std::cout << "Opening font '" << font_path << "'..." << std::endl;
		if ((font = TTF_OpenFont(font_path.c_str(), font_size)) == NULL) {
			throw std::runtime_error(std::string("TTF_OpenFont Failure: ") + TTF_GetError());
		}

		std::cout << "Creating SDL's main window..." << std::endl;
		if ((main_window = SDL_CreateWindow(
			app_title.c_str(),
			window_rect.x,
			window_rect.y,
			window_rect.w,
			window_rect.h,
			SDL_WINDOW_SHOWN
		)) == NULL) {
			throw std::runtime_error(std::string("SDL_CreateWindow Failure: ") + SDL_GetError());
		}

		std::cout << "Creating renderer..." << std::endl;
		if ((renderer = SDL_CreateRenderer(main_window, -1, 0)) == NULL) {
			throw std::runtime_error(std::string("SDL_CreateRenderer Failure: ") + SDL_GetError());
		}

		std::string text = "";
		bool update_text = true;
		SDL_Event event = {};

		std::cout << "Application started." << std::endl;
		while (!should_quit) {
			while (SDL_PollEvent(&event)) {
				switch (event.type) {
				case SDL_QUIT:
					should_quit = true;
					break;
				case SDL_TEXTINPUT:
					text += event.text.text;
					update_text = true;
					break;
				case SDL_KEYDOWN:
					switch (event.key.keysym.sym) {
					case SDLK_ESCAPE:
						should_quit = true;
						break;
					case SDLK_BACKSPACE:
						text.pop_back();
						update_text = true;
					case SDLK_RETURN:
						text += "\n";
						update_text = true;
					}
					break;
				}
			}

			SDL_SetRenderDrawColor(
				renderer,
				background_color.r,
				background_color.g,
				background_color.b,
				background_color.a
			);

			SDL_RenderClear(renderer);

			if (update_text) {
				auto text_surface = TTF_RenderText_Blended_Wrapped(
					font,
					text.c_str(),
					foreground_color,
					0
				);

				if (text_texture) SDL_DestroyTexture(text_texture);
				text_texture = SDL_CreateTextureFromSurface(renderer, text_surface);
				SDL_FreeSurface(text_surface);
			}

			if (text_texture) {
				SDL_Rect text_rect = {};
				SDL_QueryTexture(text_texture, NULL, NULL, &text_rect.w, &text_rect.h);
				SDL_RenderCopy(renderer, text_texture, &text_rect, &text_rect);
			}

			SDL_RenderPresent(renderer);
		}
	}
	catch (std::runtime_error ex) {
		app_return = 1;
		std::cout << std::endl << "Runtime Error Encountered >>" << std::endl;
		std::cout << ex.what() << std::endl << std::endl;
	}

	std::cout << "Unloading fonts..." << std::endl;
	if (font) TTF_CloseFont(font);

	std::cout << "Destroying SDL's main window..." << std::endl;
	if (main_window) SDL_DestroyWindow(main_window);

	std::cout << "Destroying Text Texture..." << std::endl;
	if (renderer) SDL_DestroyTexture(text_texture);

	std::cout << "Shutting down SDL's renderer..." << std::endl;
	if (renderer) SDL_DestroyRenderer(renderer);

	std::cout << "Shutting down SDL_ttf..." << std::endl;
	TTF_Quit();

	std::cout << "Shutting down SDL..." << std::endl;
	SDL_Quit();

	std::cout << "Finished." << std::endl;
	return app_return;
}