Weakly-supervised Spatio-Temporal Video Grounding(STVG) aims to localize target object tube given a text query, without densely annotated training data. Existing methods extract each candidate tube feature independently by cropping objects from video frame feature, discarding all contextual information such as position change and inter-entity relationship. In this paper, we propose Video-Text Prompting(VTP) to construct candidate feature. Instead of cropping tube region from feature map, we draw visual markers(e.g. red circle) over objects tubes as video prompts; corresponding text prompt(e.g. in red circle) is also inserted after the subject word of query text to highlight its presence. Nevertheless, each candidate feature may look similar without cropping. To address this, we further propose Contrastive VTP(CVTP) by introducing negative contrastive samples whose candidate object is erased instead of being highlighted; by comparing the difference between VTP candidate and the contrastive sample, the gap of matching score between correct candidate and the rest is enlarged. Extensive experiments and ablations are conducted on several STVG datasets and our results surpass existing weakly-supervised methods by a great margin, demonstrating the effectiveness of our proposed methods.