One more session working on the text box. I think it is ready.

I created a new TTF_Font variable named textboxFont and changed the position of the interiorTextBox .

TTF_Font *textboxFont;

textboxFont = TTF_OpenFont(fontResource.location, 50);
TTF_SetFontStyle(textboxFont, TTF_STYLE_BOLD);

// Inner text box location
const SDL_FRect interiorBoxRect = {    exteriorbox.x * 1.22, 
                                    exteriorbox.y * 1.125, 
                                    interiorTextSurface->w, 
                                    interiorTextSurface->h};

I modified the TextBoxHandler() function to make the cursor blink. I used the frameTime counter to achieve this effect. I might need to create another variable similar to frameTime.

void TextBoxHandler(TTF_Font *font, SDL_Renderer *renderer, 
                    struct ExteriorBox exteriorbox, struct InteriorBox interiorTextBox, 
                    size_t length, double *frameTime)
{
    SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xFF, 0xFF);
    const SDL_FRect exteriorBoxRect = {exteriorbox.x, exteriorbox.y, exteriorbox.width, exteriorbox.height};
    SDL_RenderRect(renderer, &exteriorBoxRect);

    // Textbox text
    if (interiorTextBox.isEnabled && *frameTime < 0.5)
    {
        char *temp = calloc(length + 1, sizeof(char));
        SDL_Color fontColor = {0xFF, 0xFF, 0xFF, 0xFF};
        strcpy(temp, interiorTextBox.content);
        strcat(temp, interiorTextBox.cursor);
        SDL_Surface *interiorTextSurface = TTF_RenderText_Solid(font, temp, fontColor);
        SDL_Texture *interiortextTexture = SDL_CreateTextureFromSurface(renderer, interiorTextSurface);
        const SDL_FRect interiorBoxRect = {    exteriorbox.x * 1.22, exteriorbox.y * 1.125, 
                                                                    interiorTextSurface->w, 
                                                                    interiorTextSurface->h};

        SDL_RenderTexture(renderer, interiortextTexture, NULL, &interiorBoxRect);
        SDL_DestroySurface(interiorTextSurface);
        interiorTextSurface = NULL;
        SDL_DestroyTexture(interiortextTexture);
        interiortextTexture = NULL;
        free(temp);
        temp = NULL;
    }

    if (interiorTextBox.isEnabled && *frameTime >= 0.5 && strlen(interiorTextBox.content) > 0)
    {
        SDL_Color fontColor = {0xFF, 0xFF, 0xFF, 0xFF};
        SDL_Surface *interiorTextSurface = TTF_RenderText_Solid(font, interiorTextBox.content, fontColor);
        SDL_Texture *interiortextTexture = SDL_CreateTextureFromSurface(renderer, interiorTextSurface);
        const SDL_FRect interiorBoxRect = {    exteriorbox.x * 1.22, exteriorbox.y * 1.125, 
                                                                    interiorTextSurface->w, 
                                                                    interiorTextSurface->h};

        SDL_RenderTexture(renderer, interiortextTexture, NULL, &interiorBoxRect);
        SDL_DestroySurface(interiorTextSurface);
        interiorTextSurface = NULL;
        SDL_DestroyTexture(interiortextTexture);
        interiortextTexture = NULL;
    }

    if (*frameTime >= 1)
    {
        *frameTime = 0;
    }

    if (!interiorTextBox.isEnabled && strlen(interiorTextBox.content) > 0)
    {
        SDL_Color fontColor = {0xFF, 0xFF, 0xFF, 0xFF};
        SDL_Surface *interiorTextSurface = TTF_RenderText_Solid(font, interiorTextBox.content, fontColor);
        SDL_Texture *interiortextTexture = SDL_CreateTextureFromSurface(renderer, interiorTextSurface);
        const SDL_FRect interiorBoxRect = {    exteriorbox.x * 1.22, exteriorbox.y * 1.125, 
                                                                    interiorTextSurface->w, 
                                                                    interiorTextSurface->h};

        SDL_RenderTexture(renderer, interiortextTexture, NULL, &interiorBoxRect);
        SDL_DestroySurface(interiorTextSurface);
        interiorTextSurface = NULL;
        SDL_DestroyTexture(interiortextTexture);
        interiortextTexture = NULL;
    }
}

Result